I'd get John Burke to do the negotiations for you, then you won't pay more than 300 bucks. Lots of times if the seller just wants to get rid of a piece of juk clutteiring up their garage or yard, you can get em fairly cheap. Now that you have put a rare appilation on the thing, you have raised the value in your mind. Three windows in general aren't that rare. 49 was the first year Plymouth switched to that body style, quite a few were mad in 50.
go to the magazine store and get an Old Cars Price guide, and a Hemmings looks for similar cars for sale and look at the condition guide. If it is a running
survivor (an original car that has not been restored, renewed or rebuilt) it may be worth more in that all except maintenence parts are original to the car.
The other factor is how much you can afford. But start low and see how the negotiations go, Is anyone else interested inthe car???? How eager is the seller to get it gone. Lots of things to consider beside the actual value of the car. But the price guide and hemmings will give you an idea. Also go to ebay and check out the plymouth section for completed deals on similar vehicles.
There are two on now, here is one at 2500.
http://cgi.ebay.com/ebaymotors/1950-Plymouth-Coupe-3-window-57-000-original-miles_W0QQitemZ260072260620QQihZ016QQcategoryZ6387QQssPageNameZWDVWQQrdZ1QQcmdZViewItem