Composition software, eh? Cheap and easy? How about free?
I've recently been fiddling with SunVox. It's a modular tracker, very easy and fun to use, and you don't really need any sort of special hardware. coupled with Audacity for recording/editing samples and exporting as ogg vorbis, SunVox is an incredibly powerful tool for PC, Linux, Mac, and various mobile platforms.
I've mostly been using it for noise, but it's amazing for pretty much any genre.
EDIT:
If a tracker isn't your thing, though, I'd recommend energyXT. it's $50, if I remember correctly, and has a nearly unrestricted demo version (you can't save project files in the demo, but you can export completed tracks and save presets). It's a very lightweight DAW that runs on Mac, PC, or Linux, and it's very easy to use if you know producing terms.