The standard recommendation for a 'cheap but decent' USB sound card is the Behringer UCA202 or the nearly identical UFO202 and UCA222. It's only 16bit/48khz, but the analog performance is reasonable, and it's only $20 or on sale right now for $10 at Sweetwater. It's a UAC1 device, so works on pretty much anything with a USB host port. However word on the street is they've switched to a clone of the original TI all-in-one chip, and that performance is worse than before on the new units, though likely performance is still reasonable for casual use. YMMV, but for $10 I think it'd be hard to go wrong, though performance won't be incredible either way.
The next step up is a pretty big one to the Behringer UMC202HD (you will need 2 RCA -> 1/4" TRS cables) for about $80, with 24/192 and significantly better analog. Also bigger, more complicated, and with controls you probably don't need/want, but I'm not really aware of anything cheaper that offers good audio quality.