There are lots of good platform pedals out there at various price points. Clem's got some good suggestions above. I've found that MKS makes high quality pedals for the price and he mentioned one of their models. Their RMX pedal might be my first suggestion for you. If you want to spend as little as possible for a pedal that's better than the plastic ones your bike came with, you could go with alloy BMX-style pedals like these Diamondback Bigfoot pedals -- just be sure to get the ones with a 9/16" axle.