Harry, welcome to TBN
Condition, would be key here, a well cared for 40 year old tractor is worth considerable more than one in poor shape. Fixing the AC might be cheap or expensive, depends on what is wrong with it. The older R12 systems can be converted to R134a too which is currently used.
You should look at sites such as Tractorhouse and Machinefinder to get a rough idea of the asking prices for comparison.