Over a 20 year period, it's likely to increase. Classic cars have a long track record of price increases. However, and this is a big one- just like the stock market and real estate market, there will be booms, busts and periodic corrections. I believe we are on the verge of a major correction due to investors driving up prices. But that's just short term.
Being a unique and rare fiberglas car, I think the Darrin is more likely to increase in value than the average Kaiser. It's better than money in the bank, if you bought it right.