FYI: If you wanted to convert your bike, you'd need the following -

A wheel with even dishing with a hub threaded for a freewheel and an axle long enough to fit your frame.
A freewheel
A chain tensioner (the rear derailleur will work as long as you can keep it shifted in one spot)
A single speed crankset (middle ring would probably line up if you -really- wanted to keep the same crankset)
A new bottom bracket to fit said crankset.
A new chain

So yeah, you can probably pick up a low-end single speed cheaper.