Aside from the technicalities involved, there are two other considerations:-
1. Legality of dual pricing. May be a good idea to see if there are legal restrictions.
2. Having two prices on the same site, applicable to different regions, would be very annoying for customers who were being asked to pay more - especially if it is a virtual product, where there's no packing and shipping excess. Your client could actually drive foreign business away with this policy. I most certainly would NOT pay extra, just for being a foreign customer (or vice versa). I would shop elsewhere - even if just to show my disgust for the dual pricing penalty.
If your client is intent on dual retail pricing, then it would probably be better to set up two separate shops. Configure one to accept orders ONLY from USA, and the other to accept orders from everyone else.
Besides, there is no mechanism in a standard install of ZC to cater for dual pricing on the basis you suggest. You can invoke the Pricing Groups feature, or consider the Dual Pricing Add-on:
http://www.zen-cart.com/index.php?ma...roducts_id=166