While I know iPhone users can be very brand-loyal, if the price is too high, consider buying a cheap Android phone and using it with your SIM. At the end of the trip, you can put it back in a new iPhone, or (dare I say?) change sides...
The iPhone 5 uses yet another made-for-Apple incompatible SIM size, but phone stores will have a small plastic holder to retrofit it to the sizes used by others.