First, is the wheel badly bent or just out of true by a few millimeters/1/8" or so? If the latter, the wheel can be easily straightened by adjusting the spokes. If the former, it's possible to fix but likely not worth the effort.

So assuming you need a replacement you'll need to confirm a few things first. #1, does the bike have a freewheel or cassette? This page should help you figure that out if you can't find the specs for your bike: http://www.sheldonbrown.com/freewheels.html One easy indicator is the bulge or lackthereof on the rear hub. If you don't have a bulge, you have a freewheel.

#2, does have a nutted axle or a quick release? You'll want the same type of axle on the replacement wheel.

#3, what is the over-the-locknuts dimension? This will be either 130mm or 135mm. You can measure the distance between the rear dropouts as an alternative to measuring the assembled wheel which can be a bit difficult.

At your size, a 36 spoke wheel wouldn't be wasted strength. Your $100 budget is plenty as well. If you want to run a 38mm tire, about the only type of wheel you'll want to avoid is a standard road bike wheel (and there aren't many of them available with a freewheel or 7 speed cassette hub anyway).

On to some options, assuming the most likely scenario (freewheel hub, 130mm OLD, and quick release), this 36 spoke wheel will get you back on the road: http://www.niagaracycle.com/product_...ducts_id=34651

If you wanted something more robust, you could go with a wider rim like this wheel: http://www.niagaracycle.com/product_...ucts_id=509766

Both will take a 37mm tire easily. The latter would allow you to go as wide as the frame will allow.

Regardless of which wheel you choose, use some of that $100 budget to have a skilled wheel builder (hopefully available to you at a nearby bike shop) go over the wheel before you start riding it. Evening out the tension and making sure all spokes are at their safe max tension will insure the wheel stays true and lasts as long as possible. Granted, dropping the rear wheel off a curb is a tough task for any wheel but you'll be better off if you start with a well built wheel (and unfortunately, with the machine built wheels that are in your price range you get no guarantee of proper tension). This checkover might cost as much as wheel but it'll be money well spent to avoid the aggravation of broken spokes down the road.