What do you mean by "Background music"? Do you mean score? And define "Cheap." To me anything under $500 is cheap. How in depth is your music background? Do you you know keys, meter, instrumentation, scoring, MIDI, sampling and digital audio? Or do you want to just cut and paste loops/samples? Just keep in mind that, initially, the less you know the more expensive it will become as you will have to buy sample/loop refill packs to expand your sound library.
A few suggestions - Cubase, Reason, Acid, Band-In-A-Box, Cakewalk.