Originally Posted by MSPeconomist

Is it now 20 Euros rather than US$20 for USA passports?

In these cases, it's wise to have the exact change in the required currency just in case something happens, such as a missed connection, and you want to use visa on arrival to enter the country.

At IST, the procedure that I've used is to hand your passport and a nice new USA twenty dollar bill to the guy at the visa window. The passport is handed back with the "visa" stamp and you go on your way. Just check that you've received the right passport back.