You can get the basic HD axle kit for just under 50, but unless you know how to lace a wheel, you'd have to pay a shop to do it.
I picked up a disc wheel from a local bike shop for actually exactly 50 dollars. Granted, I also called EVERY bike shop in the area to find that price. And unless you have the tools and/or cassette for it, you might have to buy a cheap single speed ring and spacers.
Absolute cheapest route is to go to a thrift shop and grab up a cheap bike that has the same size wheel. Usually you can score with 20 bucks, and that'd get you a wheel that you can use until it dies with the ragjoint. Then, you have spare parts galore! Brake levers, cables, cable housing, brake pads, bearings, forks, cranks, fenders, front wheels, chain, grips, handlebars, tires etc etc.