There are a few different options, depending on what you want to do.

1) Repair the wheel: If the rim isn't pinched, but bent, it can probably be straightened and work fine. This can be done by a shop, or coop by yourself, and would probably run anywhere from $20.00 to $50.00, depending on what was needed.

2) Replace the Rim: If the rim is too badly damaged, but the hubs and spokes (mostly) are ok, the rim can be replaced, reusing the other pieces. This would cost you the price of the rim + the labor, probably not worth the cost unless you do it yourself, but probably in the $100.00 range.

3) Replace the rear wheel: A bargain wheel can be found at your coop, bike shop, ebay or wherever for less than $100.00.

4) Replace both wheels: Most bargain wheelsets begin at $150.00, maybe you could sell the front wheel cheaply to lower the cost some, but probably not by much.

The wheels are pretty cheap to begin with, if 1) doesn't work and you're not comfortable with 2), then you're probably better off with 3) or 4).