If you're serious about owning a Miura, I can suggest a number of places you can do some research on the cars.

http://www.qv500.com/indexlamborghini2.htm

http://www.lamborghiniregistry.com

http://www.miuraregister.com

http://www.web-cars.com/miura/index.html

There are many things to consider when buying one, so one of the best things you can do is to educate yourself on the variations. As a general rule, higher serial # cars command more money, but so much of the price is linked to condition.