Absolute cheapest way if you have tech. skills to get one subsidized from a carrier in the West and unlock it yourself so you can use it both (in GSM mode if it has both).

Other way is to buy it online locally and get a 900/1800 phone (absolute cheapest) 2G GSM phone or quad-band phone and you can use it in the US/Canada/Americas upon insertion of a AT&T/T-Mobile SIM card.

If you want 3G (UMTS/HSPA+) you want 2100MHz abroad and 850/1900 added for Americas and 900 added for similar to Americas (Europe) for in-building low-band coverage.