There is a lot of cheap mobile. The best mobile and cheap model you can find on Nokia and Samsung. For example I bought a Samsung SGH-E250 for my mom only for 50$
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.