I would think that an ITC car would be the cheapest to run -the tires are smaller and cheaper to replace. Plus - they feel fast when you are wringing the crap out of 'em when the exhaust is mufflerless. But some of the ITC cars in the GCR are getting a bit old.
VW Rabbits/Sciroccos are quite durable.
The early Honda Civic/CRX is also durable and both have good resale value. I'm not sure where the Ford Escort falls in the IT ranks - the later Mazda powered cars are cheap to purchase and there are TONS of parts laying around - just find out what transfers from the Mazda partsbin. Toyota has tons of varieties of compacts that are durable as does Nissan. I wonder how a GM J-car(Cavalier/Sunbird) would do cost wise? Mopar L bodies are another wildcard. I've seen 'em - but I don't know how they do on the track.
Make sure you can get a Limited Slip Diff. for what you buy.
Check what is on the grid at the Run-offs or June sprints and you'll see what is competitive.
I'm eyeing up Honda Accords for an IT possibility - they seem to be doing well in ITB.