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FWIW - the ragtop conversion parts alone are $4-6K (depending on how many used parts you can find) without installation labor - and sawzall blades for removal............
Yep...but, if done correctly, you have valuable coupe parts to sell and recover some of the costs.
Gunner -- the answer to your question is easy:
Supply and demand -- having a (well done) convertible in the 76-82 "gap" is something in pretty short supply and there will always be demand to keep the value of your car above the value of an equal condition/mileage 81 coupe.