a quick search on wikipedia resulted in the following:
Free and open source systems
I do want to stress I have no experience with any of these, but they fit the bill of 'affordable' pretty well, I'd say. :p
You could also try your hand on Renoise. It's not like, say, FL Studio in ways of building up music through patterns, but I've heard absolutely amazing results come out of it. It's relatively affordable at only 58 euro's for the full, legal version.
Just be ready to go through a gradual process of learning. I tried my hand on it once and it seemed like way fun, but it was so different than what I was used to, considering it's a tracker and I've been using FL Studio for almost all my producing lols.
Also, FL Studio is definitely not 400 bucks. The producer version is like 200 afaik and that provides mooore than enough. You can't buy older editions either, you buy the version (lite, producer, XXL) you want and on purchase are subscribed to a lifetime of updates on the products (I.E. FL Studio) you bought, so you can upgrade from 8 to 10.. for if you have been living under a rock or whatever. :p
Hope you get your hands on some nice software. Don't settle for crappy, overly simplistic stuff cause it will probably be a pain in the ass!