I guess it depends on the fish and what you plan to do with them. If you're serious about breeding and showing bettas, you'll be more willing to fork out the dough for a more expensive pair, so that you will have a better chance of getting some show quality stock from them, or fish that will bring in a good profit for you when you go to sell. If you just looking to spawn and sell on the side, or give away (for the experience of breeding), spending the big bucks isn't really necessary. It all depends on your focus, and your budget. Its okay to start out on a smaller scale and with less expensive stock. It would be awful to spend a large amount of your money on your first spawning pair, only to have one of them die in the process. Starting out with a less expensive pair is the way to go, especially if you're on a budget .
Now, I can be a cheap-skate, I'm willing to spend $40, $50, $60, maybe even up to $80, but only if I'm getting a pair, not a single fish. For a single male, depending on the fish, I'd probably only go $30 or $35, for single females, less. For wild-type bettas, it all depends on the species. The next species I'm looking to get have been running about $40 for a pair, which I'm more than willing to fork out when I have the space, as my focus is mainly on wild-types and building up a good SMP stock.