If it's not running, $100. If it runs, and has decent compression and no obvious problems, perhaps as much as $300-$500.

Unless you have a particular attraction to steel wheels, they are a major PITA. The ride will liquefy your kidneys, you can't take it on the road (therefore, no parades) they tear up the lawn, and traction is not great. You'll likely want to change to rubber, which adds to the expense of "restoring" the tractor. Also, a transmission geared for steel will make an already painfully slow tractor even slower.

Having said that, I blew $4300 rebuilding one on rubber (Nearly $3000 of that on a total engine overhaul. Only thing original is the crank, cam, oil pump, carb, governor and a few miscellaneous nuts and bolts.) And I'd do it again. Most fun you can have with your clothes on...