Hi Jinks
Jammer Pro starts at $129 http://www.soundtrek.com/catalog/
Band in a Box starts at $88 http://www.pgmusic.com/bandbox.htm#1

Bear in mind your computer needs to have certain system requirements ( should be listed on the sites), and you'd need either a software synthesizer or a soundmodule or synth midied to your computer.

Basically you type in a chord progression, choose a style & the program creates a backing track. (They actually do more than that, thought).
Don't forget there's free demo's on site, so TRY before you consider buying.
best wishes
Rikki