I'm pretty sure that it is because you can pair one male to a lot of females so you want to spend more and get more of those, but a female can only have one clutch at a time, and one per season, so you want to get the most out of your money, which means that if you have an expensive female, like a banana spider or something, and you breed her to a normal male, you only have one opportunity to get a clutch with banana spiders and for more clutches like that, you could either spend a ton on more banana spider females, or you could get a bunch of normal females and one single banana spider male. Does that make sense? I hope it did lol