I don't want to give away all my secrets, but here is the big one... If you buy right, and can do mechanical work yourself, motorcycling doesn't have to cost you much if anything.
Here is a short list of bikes I've bought for under $1500 (some well, well under):
1979 CB650 6500 miles
Dream 305 (3 of them)
Hodaka Ace 90 nearly mint, just didn't run :-)
1985 FJ1100 17,000 miles (this was an absolute steal)
There are lots of bikes out there for under $1500, you just have to keep your eyes open and have cash in your pocket if you stumble across a deal.
Darn, gave away another secret. :-)