I find cheap, but decent, MTB wheels all the time, rarely pay over about $5 each. Garage sales, Craigs List, thrift stores, and wherever. No need to buy new. If you are lucky, you will find a used wheel complete with either a 7 speed cassette or freewheel. About half the wheels I find come with tires, tubes, everything. Just depends how hard you want to look. OK, I look pretty hard, so maybe you will have to pay $20 a wheel.
More than once my wheel pickups came in the form of a complete bike, perhaps one that had paint damage, or other issues (but good wheels, crankset, tires, etc.) A $5 bike often can provide a thrifty guy a lot of good parts: saddle, grips, tires, tubes, wheels, derailleurs, levers, etc.
Older MTBs tend to have minimal value, so they can make a great deal for an end user as something to ride. And the good ones often sell for just a few dollars more than the bottom end/crappy ones.
I run Nashbar 26 x 1.25 slicks on my MTB I use for road riding. I swap out the wheelsets with one that has trail tires on it for trail riding (hey for $5 a wheel, why not???)
Here is the bike with slicks:
Same bike with trail tires:
