Antique tractors were in vogue about ten or fifteen years ago, but with the influx of imported compact and utility tractors equipped with four wheel drive /assist , power steering, air conditioned /heated cabs, factory loaders with quick attach buckets, and hydrostatic transmissions, interest in the old machines seems to have waned, and prices have receded.
I am not an expert, but I do think $2500-4000 would be reasonable. If the tractor has been on the market for a while, the owner might jump at a cash offer near the lower end of that range.