[DeanLag]

Every month? Thats a bit too much *imo* but if you can afford it, why not.
However, if you can afford that everymonth, why not post in Paid Services? More people will be willing to do that as most coders don't really do volunteering because, maybe, they're not going to use the mod itself on their board =p

However, theres other alternatives:
I kind of use the same method to try to increase traffic to my board (Although I don't give $15+ prizes =p). I use vBcredits with vbCommerce (available at vbcredits.com). When users post, they earn 'forum points' (Can call it $, potatoes, cookies or whatever currency you want it to be called) and then they use the currency to buy tickets in the lottery.
The lottery system is quite versatile. You can run multiple instances of lotteries and you can also schedule an end-date. The winner will be shown in ACP and the winner will be PM'd as well (Everything automatically!)
Each ticket increases their chances of winning the lottery (duh) and you can price the ticket at any amount. Might want to put the prize to 1 'forum point', so that the next lottery so that the winner doesn't use the winnings to buy more tickets.

Moreover, with vBcommerce you can buy items in a 'shop' and the items will be displayed on their postbit, and I assure you, forum users LOVE to differentiate from the 'average' member.

I'm not associated with vBcredits in anyway but I'm really glad and grateful that such a versatile product is free to download and use. Might want to check on the tons of features the mod offers.