How much is a Marx 3000 Train Set worth?

It depends WHICH MArx... Most are Tin, or diecast. The most valuable would be in MINT condition, some of the model Include the TIN friction set (all lithographed multi colored) M10005 set which could have as many as 7 trailing Named City rolling items) also an unusual find is the 1829 engine. Condition is paramount in all cases for Marx trains.