You can find copies of the "Old Cars Price Guide" at most major chain newsstands. It comes out quarterly. It now covers cars into the '80s decade. Use the values in there for reference as each area of the country might be different. Also most people overestimate the condition level by one notch too high, so keep that in mind too.
Those LeBaron convetibles were neat cars for their time and also started the current convertible trend--as everyone else had stopped building them in the later '70s, at least on this side of the water, except for some conversions of Brand X luxury coupes in the middle '80s.
Unfortunately, I don't suspect the value of those vehicles is what it might be. At this point in time, all things considered, if you like it, then buy it and cherish it for what it is but don't suspect it to ever be an appreciating collector vehicle as other older vehicles are. BUT it can be a pretty much "flat depreciation" vehicle too so that takes some of the bite out of owning one (i.e., you can drive and enjoy it and keep it nice instead of it being a garage queen). Be sure and get a good insurance policy on it too, if you get it.