Maybe your insurance company is asking "what did you pay for the tractors when you bought them"?
Deere likes to support the used prices of their tractors. Most older Deere tractor in usable condition still sell for their new price or higher so don't sell yourself short.
If your insurance is like antique cars your premiums are based on the insured value, up to the current appraised value. I'd take some pictures to support their current condition and then insure your tractors for their current replacement values. The insurance company will have the serial numbers, they can research the original list price if they want to.
In 1971 dad bought a 2020 gas loader tractor. There was a laminated "window sticker" total just over $6500 list price. Options that I can remember were dual hydraulics, rack & pinion axles, heavy cast wheels, engine side frames, continuous running rear 540 PTO, telescoping 3PH links, and 14.9x28 & 7.50x16 tires. Live PTO was expensive, another $600, I'd guess a diesel engine would have added at least $500.