It depends on what service provider you have. Straight talk from wal mart is bery inexpensive and they have a phone for 30.00 which is pretty reliable. Although it depends on what you want out of your phone. Smartphones are great if you actually use all the applications.
Generally the phones you can buy with some of the pre-paid services can be very inexpensive. A number of them offer phones for as little as $10, and while they may not have all the functions of a smart phone, they work very well for simple phone calls.
The cost efficient way to have a cell phone is a pay-as-you-go phone. There are many brands, but T-Mobile has one that costs $19. 98 and allows you to pay a daily fee for only days it is used. It may not be a 'fancy' phone, but has everything needed.
In my experience from working in cell service customer service and buying new cell phones every year my opinion is that Nokia is the most dependable for the price. They are durable and inexpensive compared to other brands. Also Motorola has been very durable when I have had them, especially the Razor.
When you start a new contract with your wireless carrier there are always several options that are cheap with a rebate, cheap in general or are free with the plan. The phones depend on the carrier. If you don't rough your phone up the free ones will last two years.