The iPhone itself may be a small part (depending on your budget) but wireless plan costs also vary widely. I know that UK/EU plans are substantially cheaper than the US where many pay $150+ per month for a couple lines.
In Taiwan, for example, I got a 3G phone plan for $40 month-to-month, unlimited data, and consistently ~20mbps download speeds.
In developing countries like Thailand or Central America, I can get a 3G card for 60 days and 1-2GB for less than $10. Speeds obviously aren't as good as Taiwan, but the basic access to Internet is way cheaper.