I had been looking to buy one for my Boy Scout Camp for a few years and missed a few reasonable. Cheapest I recall was GSA in Kentucky forestry for $350. Most sell for a $750-$1000 formerly GL and now GP or just under but recently a few GSA in NV went for $1500.
It really depends on which tank, is the big lid ok?, old style frame, newer style? dual wheels & fenders? pump platform? Location
Reason prices are all over the place is also a construction company, farmer, or contractor will pay gladly easily $1500-2000 but that is typically out of hobby range for limited use.
Cheapest ones are usually ex forestry or fire fighting ones at GSA auction