Thanks Mike, adjusted for inflation, $4500 seems about right. (Hard to believe I've had my car 8 years.) There are plenty of variables affecting the prices of 40 year old cars, but if you could find identical condition cars, the attraction of the soft top is indisputable. That was the original question.
I have an electric hoist in the ceiling of my garage and it takes under 2 minutes to be ready to roll and about that to put the top back on. And, because I know the coupe is less desirable than the roadsters, I don't feel guilty making modifications that take it away from it's Germanic purity. Also, after riding in Jamie Howard's excellent 230 in France last September, the soft top is at best "fiddly". So, to me, the car has a value that it probably doesn't have to most.
That said, if I buy another some day, it will be a roadster/coupe (both tops).
As to the toneau idea, there are manufactured cockpit covers sold for the car, but they don't fit the California Coupe. I bought one and it expects to be clamped into the top compartment cover, so it comes up about 5 inches short of covering the tub for the back seat. I gave it away at the PUB last time. I haven't gotten around to visiting our local sail maker to have one custom made.