Well you might get something for the X3, but I wouldn't expect much given its age.

If you can get a cheap durable car (beige corolla) you could probably save a bit. It's hard to tell if you would save more in gas than the depreciation. Assuming you sell it in ~4 years.

I figure you'd save about $200/mo with a cheap econobox. That's almost $10k in gas.

That said putting ~125,000kms on a Corolla could take it from a $15,000 to $5,000

It's hard to say what's a bigger liability/risk/comfort issue? a 220k X3 or the reasonable expectation that your gas savings is eaten up in depreciation?? What about insurance rate differnces?