Depends on your goals and budget. On the high end, you have electronic trainers over $1200+ and can get one as cheap as around $100 new.

I would spring for at least a decent fluid trainer. In the northern hemisphere, spring is coming, so you should be able to pick up a decent used fluid trainer on Craigslist for cheap. I'd look for a CycleOps or a Kurt Kinetic. Otherwise the $100 run of the mill Nashbar or Performance Bike fluid isn't too bad.