It depends if you have the "seal mechanism" bearings. The seal just makes it harder for water to get at the bearings. You just need to go to the LBS and get some ball bearings of the right side, take out the old bearings, replace with the new, clean the grease out, put in new grease. With grease and bearings, you are talking less than $10 (or so I think).
If they are cartridge bearings, you can buy the cartridges and replace them. I don't know what they charge for cartridges these days.
The thing about cartridges is they tend to last a long time. You just wait until they run rough.
With loose bearings, waiting too long can wear the races and cones which is not good. So you need to do it every 5000 miles or so.
Ok I know nothing about wheel bearings. How do I tell what type I have? My bike is a Giant Nutra which is an older model hybrid I believe they call it.