I don't know if there's a rule of thumb for the consultants preparing the PEA to select a gold price as I've noticed a range of values quoted. I do know the NPV of a project has a very high sensitivity to the gold price so you need to minimize this project risk if you want to raise financing, particularly from banks. This fact pushes companies to select a conservative value for the gold price yet still gives a good NPV and IRR. Just google search PEA gold price and see the wide range of results. Here's a recent example with a $ 1600 gold price:

http://www.mining.com/2012/03/14/woulfe-mining-sheds-8-5-despite-positive-gold-mine-pea/

I doubt they would get financing.