In real nice shape and with inside 6 way full hydraulic blade - they still bring $7000-$9000. Many "beaters" sell for $2500-$3500. I rarely see them in truly excellent condition.
If it's an original 350C - major wear areas are the torsional isolator for the reverser, wet steering clutches slipping when hot, worn out floating mounts where the cross-bars attach to the track frames, cracked side frames and steering clutch housings , and the BIGGEST problem - the reverser case separating from the transmission case. The two cases are held together with four fasteners. Two nuts on studs on top, and two bolts on the bottom. If any are missing, loose, etc. - walk away from the deal.
There is no cost-effective fix.
There are also 350Cs around that were bought back by Deere Company, completely rebuilt and updated through the Deere "Certified Relife Program." I suspect that sort of C could be priced very high. Deere was charging $60,000 to do it - and that's not counting the cost of the 350C being traded in.