You can take your wife to planned parenthood and get either a five year or ten year IUD. I paid for my girlfriend one to get one and it only cost me $250. At that price, your break-even is 3.1ish months.
One interesting thing about the IUD is that it can cause a chemical abortion. I know a lot of mormons who use an IUD and it seems generally accepted. Maybe ignorance? The IUD works in two ways, one way is to thin out the uterus so the embryo cannot attach if the woman gets pregnant. Anyway....