I know how you feel... the software is so expensive you dont have many options for people who just do it as a hobby like myself, and just want to learn to use it.
I dont know what kind of music you want to make, but be aware that you wont have much in the way of software instruments to make music with out of the box. If you plan on recording audio and adding fx then cubase may be a good choice.. if your looking for software synthesizers (VST plugins) be warned cubase sx only comes with about 5 pretty limited synthesizers.. you might be disappointed.. you have to use 3rd party plugins.
There are tons of sequencers out there.. might try as many as you can before you go spend the money.
Another free one is
http://ardour.org/
Its near professional quality, but only runs on Mac OSX or Linux.
Also, some good music can be made with a tracker program called renoise.. its a midi tracker, it doesnt record audio though. Its only 50$ and pretty kick ass.
In the end though I am doing the same and dropped $1000 on Logic 7 Pro. Should be getting it tomorrow, you probably wont see me on the forums for a while after it arrives;P