The 15$ is a set amount and it is actually a LOT cheaper per month than every other site worth donating to. Personally I wouldn't like seeing smaller amounts, for the same reason as having a 2$ for a week would be a bad idea. People would donate, leech everything in the various elite/hidden sections, never post and then never donate after the 2 weeks is up.
What we want is for donators to stick around, talk in the SB, maybe make a few friends. I've been talking in the bawx pretty much every day for 2 years, I'd be glad to donate 15$ every 6 months just to talk to people in it. Reality is, most of us have grown attached to at least a few people that we've met by either donating / getting to 80 rep. Shorter donation periods would be a no for me, I want people to stick around for more than 3 months.