Fixed Gear Bicycles

Uncle brought home a busted bike, and I wanted to see what I could do with it in terms of having a spare in case my main one has any complications.

That being said, it's a shift gear, both tires inflated, derailleur has been ripped off, shifters are completely gone, and half of the cassette has fallen off (3 of the 6 discs or w/e you call them are gone).

That being said it'd probably take less effort to simply make it a fixed-gear than trying to get it back up to speed as a shift gear (could probably get a used bike cheaper than it'd be to get it back to a fixed gear).

Has anyone here ever converted a shift gear to a fixed gear? Just wanted to see how expensive of a process this is and if it'd even be worth doing. (as well as how difficult it is)

Has anyone ever had it done in a bike shop? If so, how expensive was it there.

Alternatively, if it's not worth doing, could just give the frame to the bike shop in exchange for a couple of new tubes/tires and shifter adjustments on the Cannondale (since they do things like that)

I have built quite a few bicycles and unfortunately the first thing I do these days is replace to bottom bracket (it's almost always in need of a replacement on salvaged bikes) which probably means you're taking it into a shop no matter what. Converting a geared bike to a fixie isn't a difficult task, but to have something that's enjoyable to ride you want to buy a new rear wheel with an appropriate gear ratio to your chainring. A good rear wheel is pretty much indispensable when riding fixed.

If you live in a city with a DIY bicycle shop/co-op (most large metropolitan areas have one) you can always take it down there and try to salvage parts, but due to the dangerous nature of riding fixed gears you probably want to make sure you have new components that will be responsive.

ETA: And to answer your price question, I think my rear wheel on my most recent build was $120 (it's a flip flop hub, fixed on one side, single speed on the other) plus the tube and tire, and a bottom bracket replacement is usually like $30.