The vintage P51s are priced all over the place from $60-7500 depending on the features of that pen....$7500 was a pen that Cartier commisioned to sell in their stores.

Their past popularity makes supplies of low to mid ranged models pretty plentiful.

P51 is "out of print." I find reliable P51-inspired writers for $2-4 a pen so I have no desire to try and buy a vintage one that might not work.