In 2005 I was very close to buying a 73/74 Europa (was looking at two different cars) and they were in excellent original condition with proper maintenance records and low miles with only 2 owners. The pricing was close to $12K each, which was strong money at the time. My local Lotus dealer (Sports and Specialist Cars) convinced me to buy a new Elise instead.
So here I am, purchasing a 72 Europa twin cam in 2016 that requires a full restoration. The price for a running car but needs lots of work is in the $10K to $15K range today. I figure I'll have about $30K+ into my car when I'm done (including the price of the car) and I anticipated I would be at the top of the market. But I build my restoration projects for me and it won't be concours level because I don't leave them stock (more power, better suspension, removal of all federal emission and brake stuff).
Bottom line for me, if it's a very good car there is always a buyer. So I'm not surprised that Europa's are now in the $30K+ range, but it took a long time to get here (I've got an early Elan too that has been a slow tick up on valuation, but its steady). More importantly when I'm ready to sell my cars they will hold their value and I'll get my money out (while having the enjoyment of driving them).