A buy bet will give you more bang for your buck - that is, you can decrease the casino's edge and take home a lot more of their money. When you place a bet you do not get the number's true odds, unless you buy the bet. You need to make this extra dollar or two turn from an advantage for the house to an advantage for you.
Lets look at the four and ten.
And, some casinos will only collect the vig if you win. These methods are not normal procedures, so you have to ask before you make your bets. Buy bets are almost always more profitable than place bets, except for the six and eight. If you don't know how much to bet, just ask the dealer, and he'll be glad to help.