It is not so much resale value as "replacement cost" and that is up to each person. In this price range, $7-20,000, it is hard to think of many other cars that "hold" the value, are cheap to fix (parts are ridiculously cheap)and draw so much attention, "What is that?"
I went out trying to find a relatively new one that had been allowed to decay. I bought it at a good price, I am adding a lot of "sweat equity" (which you should not value but just enjoy.......or not), and I will be driving without depreciation in my mind. I have a high mileage Boxster which costs on average about $2.50 a mile ( I drive about 10,000 a year) factoring in routine maintanence and part replacements $$$$$$$$$$$$$ not counting gas it is $2.00 a mile on an 80,000+ Boxster, a 914 which fits in a catagory like the Speedster and the Speedster. The Speedster AND the 914 cost $260 a year to insure 'together' and I am allowed 3,000 mile on each. The plan, keep the Boxster for when I need airconditioning and use the 914 and Speedster for "free miles" or at least darn cheap ones.
By the way, 914's never seem to go up in resale even if they are magnetic. They just keep running and running and the parts are as available as any Type 4 VW.
Just drive it and smile when people envy it.