This is already happening (depending of the country, your skin color might also make the price go higher), tourists typically pay more for a lot of stuff than what a local would pay. Though, sellers are usually a bit more subtle than "Hey, give me your passport, so I can tell you how much it costs". This is how a lot of people make a living in a lot of places. In the tourism industry, I think it's completely understandable and reasonable to base a price based on what the buyer is willing to pay, more than what it really costs.