A 210 with no dataplate is worth a fraction of its value
An aircraft with no data plate is only worth what a scrap yard will pay you. From the link I provided earlier.
A data plate is needed to legitimize an airplane. Today the regulations require that each airplane manufactured prior to March 7, 1988 have—in addition to the inside-mounted data plate—the builder's model designation and the aircraft serial number externally on the fuselage either adjacent to or aft of the rearmost door or near the tail surfaces.
Since it's not in Cessna's interest to have a "hot" data plate legitimize an airframe it had no part in producing, the company will only issue a replacement data plate under certain conditions.
A Cessna data plate replacement letter issued on Dec. 2, 2002 lists three steps to getting a replacement data plate. First the owner must provide Cessna with a notarized letter describing how the original data plate was lost. The owner must also provide a letter from the local FAA Flight Standards District Office (FSDO) that states that the airplane is "as represented."
Applicants must also include a letter from a Cessna Service Center (or a Cessna Service Engineer) stating that the airplane: (a) meets the TCDS and (b) that all mandatory Service Bulletins are accomplished. The letter also states that "Modifications such as winglets, spoilers, STOL kits or any others are not installed." Once these letters are obtained they are sent to Cessna along with a check for $404 and a new data plate will be issued.