Buy Bets allow a gambler to be paid true odds. To receive those odds, the gambler pays a 5% commission on winning bets. Buy Bets are made on one of the “point” numbers: 4,5,6,8,9, and 10.

1. Craps Lay Bet Vig
2. Craps Buy Bet Vigo

That means there are six different buy bets: The Buy 4, Buy 5, Buy 6, Buy 8, Buy 9, and Buy 10 wagers. As an example, to win a Buy 8 bet, the shooter needs to roll an 8 before they roll a 7.

At 3:2 Buy odds, your winning Buy bet wins $75 minus a $2 vig (i.e., the vig on the $50 bet amount is $2 rounded to $2), so your net win is $73. In this example, the Buy bet nets you $3 more than the Place bet. The Buy bet becomes a bad bet compared to the Place bet if the casino requires the vig up front. Made directly on 4, 5, 6, 8, 9 or 10 at any time. A 'Buy Bet' is a wager that the number bet will roll before a 7, similar to the 'Don't Pass Line' bet. A commission of 5% is charged on a 'Buy Bet', and on the amount that could be won on a 'Lay Bet'. The payoff odds are shown in the accompanying.

The Buy bet (and its cousin, the Lay bet) may be considered “good” or “bad” depending on whether you pay the vig up front or after a win. Let's look at Buying the 4 or 10. The casino advantage is a reasonable 1.64% if you pay the vig after a win, but jumps to an unreasonable 4.76% if you pay it up front.

Different buy bets have disparate payoffs. The Buy 6 and Buy 8 bets pay out 23 to 21, while they have a house edge of 4.76%. The Buy 5 and Buy 8 bets pay out 29 to 21, while they have a house edge of 4.76%. The Buy 4 and Buy 10 bets pay 39 to 21 and have a house edge of 4.76%.

Players might wonder why the house edge on these wagers is 4.76%, if we said earlier that these wagers pay true odds. That has to do with the 5% commission paid on these bets, which produces a house edge for the casino.

I'll let one of the math gurus put the math out here for you, but the part you're missing is when that $1 vig is collected.
On the 5 & 9 almost every casino takes that $1 up front. So you are betting $21 to win $30 (plus it will cost you another $1 to put the bet up again). However, that means every time you lose the bet, you are losing $21 instead of $20. If I remember correctly,
the HE on the Place 5,9 is 4%.
The HE on the Buy 5,9 is 5% when paid up front.
The HE on the Buy 5,9 is 2% when paid on win only.
So you will get paid better when it hits, but you will lose more when it loses, and the extra losses will overcome the wins, resulting in less money for you overall.
In Tunica and Biloxi, MS they do allow you to Buy the 5 & 9, and only pay the vig on the win. In this case, it does pay better to take the Buy bet, and in fact, the casinos all make it an automatic buy for you. It is a little disorienting the first time you make a $25 Place 5 bet, and the dealer pays you $36, instead of $35. 'Uh, excuse me sir, but you just paid me $1 too much.' 'No sir, that is the correct pay for a Buy 5.' 'Uh, I don't Buy the 5, that is a bad bet.' 'Not here sir. You only pay the vig on the Win, same as the 4/10.' 'Sweet murgatoids, I love the rules here!'The Buy 4 bet is essentially a bet that ends when it is resolved. A 7 you lose the bet. A 4 rolls, you win the bet. If you want to make the bet again, a new vig will apply. You only pay the vig once, per bet. If the vig is collected up front, that means you pay $1 when you make the bet (ie you pay $26 to make a $25 Buy 4 bet). If the 4 rolls, you will get paid $50. But you now have to pay the $1 vig again, IF you want to have another Buy 4 bet of $25. If you don't want to pay the vig, you can get your $25 buy bet back, and the total amount you paid was $1.

Craps Lay Bet Vig

When you get to the table, ask the dealer if the Buy 4/10 vig is 'up front' or 'on the win'.
What does this mean?
Up Front on the Buy 4:
You want to Bet $25 to get paid $50.
So you throw the dealer $26, and they put $25 on the 4, and will put the BUY chip on top of your bet.
If a 4 is rolled, you will get paid $50 (and you would get your $25 Buy bet back). But you probably want to make the bet again, so you owe them another $1 to Buy the 4 bet for the second time.
Vig paid on win Buy 4:
You put $25 down and tell the dealer you want to 'Buy the 4 for a quarter'.
If a 4 is rolled, the dealer will give you $50 and ask you to throw him $1. (so you got paid $49).
Your $25 bet is still on the 4, and if it hits again you will do the same thing ($50 paid and you give them $1)

Craps Buy Bet Vigo

So you are only paying the vig, if the number was rolled. If a 7 came out after you made the first bet, you would have lost the $25 (but not the extra $1 because you did not pay the vig since there was no win)
The vig is only charged on the Buy Bet amount, and you need to ask how they handle their rounding, if you want to really minimize the house edge. For instance, on a $20 bet, the vig is $1 (20x 5%). On a $25 bet the vig is $1, even though it should be $1.25 (25x5%). Want to get a bit more of an edge? Buy the 4 for $30. Vig is still $1 in most places, even though it really is $1.50 (30x5%). Make a bet of $35 though, and many places will charge you $2 vig.
Be aware of your dealer and others around you, when you are making the bets. I like to bet as much as I can before I get pushed to the next vig price point, but that can make for some odd numbers. If I am by myself on my end of the table, and the dealer doesn't seem to mind, I will bet $30 or $35 on the Buy 4, if it is $1. However, if the end is full, I will stick with the more standard $25 bet so the dealer doesn't have to think quite as hard (even though an experienced dealer will not have an issue with either bet.)

I think I got it now. But I have 1 more question. So by what factor does the house advantage increase or decrease if they charge commission ONLY for wins? So is it always better for casinos to charge commission after a win? So the house advantage percentage calculations are usually assuming that casinos charge commission up front? Thanks.

The commission charged only on the win is an advantage to the player.
Looking at the Place 5 as the example.
If you place bet the 5, the house edge is 4.00% (I direct you to WizardofOdds.com for the math)
If you Buy bet the 5, and you pay the vig up front the house edge is 5%. (The vig is 5% and you get true odds)
If you Buy bet the 5, and you only pay the vig if you win, the house edge is 2.00% (I'm sure a math guy will jump in)
Why the difference?
Let's bet $60 on the 5
Place 5 pays $84
Buy 5 pays $90
If the vig is up front you actually put up $63
If the vig is only on the win, you only put up $60
Let's see what happens if we played it 5 times, and had 2 wins and then 3 losses.
The lines below are the betting amounts and the wins (to make it easier to understand, I will take the bet down each time, and then put it back up.
Vig up front:
(-60b-3v)+90win +60bet back = $87
(-60b-3v)+90win +60bet back = $87
(-60b-3v) = -$63
(-60b-3v) = -$63
(-60b-3v) = -$63
net = 87 + 87 - 63 - 63 - 63 = -$15
Vig paid on the Win only:
(-60b)+ 90win - 3vig +60 bet back = $87
(-60b)+ 90win - 3vig +60 bet back = $87
(-60b) = -$60
(-60b) = -$60
(-60b) = -$60
net = 87 + 87 - 60 - 60 - 60 = -$6
So, you can see we win the same net amount ($87), whether we pay the vig on the win or up front, BUT, on the rolls where we lose the bet, on the vig up front, we lose an extra $3 each time.

Thanks for the detailed example. So if the vig is charged on wins only, is there a fixed factor as to how much the house advantage is decreased by or does it vary by the point number?

It varies by the point.
The place 4/10 HE is 6.67%
The Buy 4/10, vig paid on win, is 1.67%
There is no point buying the 6/8.
The Place 6/8 HE is 1.52%
The Buy 6/8 HE, vig on the win, is greater than 1.52%