Actually, I should have mentioned that smartphones can also require additional services which add additional charges to the monthly bill. For example, as AT&T notes: "Please note: Data plan for iPhone is required for the life of your iPhone service and cannot be removed in the future." Their data plan costs $30 per month on top of the minimum $40 per month voice plan and you have to sign a two year contract.

Therefore, if I'm looking for cheap service I will go with a cheap phone to avoid additional service charges, as well as because the phone is cheaper in the first place.

One other thing, the cheapest service is a prepaid plan but most carriers don't even offer data service with a prepaid plan which defeats the purpose of a smartphone.