What's a restored '66 worth, included bigger, newer engine (1500 cc?)
This site has pretty solid info for checking the value of used & collectible cars: http://www.nadaguides.com/. Their pricing depends on ZIP code, and you'll need to tell them whether your Beetle is a sedan or convertible. Unfortunately, adding non-original parts to an older car makes determining its value harder, because the car's no longer original. The best determination of its value will be the price you manage to get for it.
Average retail value is somewhere in the $5000 range, maybe a little more if its completely restored and in great shape.
Get More Money for Your CarBe seen by 20 million shoppers. Create Your Free Listing
Search Volkswagen Beetle Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Volkswagen Beetle Experts