How much to spend on a USED Mac that will run GarageBand?

Friends of mine are looking to buy GarageBand as a Christmas present. But they need a Mac, too. They know nothing about Apple products. I know *this much* more <hold fingers about 1/4" apart> than they do, but enough to ask here.

I know they could buy any new iMac and they'd be fine, but:

1) What would be the minimum system they should look for?

2) How much could they expect to pay (reasonably) for it?

I found these specs on Apple's website, but not knowing Apple products, it doesn't help me too much:

* Intel dual-core processor

* 1 GB or more RAM

* Older PowerPC G5 and G4 computers will not be able to play back as many tracks

* PowerPC G4 and G5 computers will benefit greatly from having > 1 GB RAM

Thanks.