As far as I know GarageBand only supports stereo output; you can record from multiple inputs on a multichannel interface at the same time by selecting different input channels for different tracks down in the Smart Controls area, but output seems to be just L & R.
As far as other cheap software that can do multiple assignable outs... If $200 is cheap enough then Logic would be the obvious choice—it has everything GarageBand has and much much more (GB is basically Logic under the hood with a vastly reduced feature set, so the look and feel should be familiar). If $200 is too much then you could check out other free DAWs like Studio One Prime, Pro Tools First, and Tracktion, or the very inexpensive ($60) Reaper. You may have to do a little homework to confirm multi-output capability, though according to the manual Reaper seems to support it, and AFAICT Tracktion and Studio One Prime do as well—of course you can always d/l and install any of the free DAWs to verify.
But if you're already used to GarageBand and $200 is within the budget, then I'd go with Logic—you can open your existing GB projects in Logic and just continue working (once you've set up the desired i/o routings)..