I don't know what they cost, but they are outrageously expensive because of the cost and lack of parts. Break something and it costs a fortune to get it fixed. The Miss America P-51 Unlimited racer has been down for a couple of years for repairs.
Here's a P-51 for sale
http://www.mustangsmustangs.net/p-51/p51sforsale/4413016mar2003.shtml
Funny thing though, I had a post on the aircraft forum about a MiG-21 and someone commented that it is actually cheaper to buy (not maintain and keep flying, but just buy) a jet than a WWII bird. FWIW there are currently 35 MiG-21's registered in the USA!
Rik, to seriously answer your question, I think the reason I build models is because I love planes and bringing one to life from plastic and paint is about as close as I'll get to them. I've flown several civilian planes, but I've never flown a warbird. As a pilot I can only dream about what it's like, and building them and seeing the details and learning about them is great.