The absolute cheapest way (I can think of) is set up a system yourself.
By a contract to some cell phone provider and hook this up to your linux computer.
http://www.dealextreme.com/details.dx/sku.12057
Here is a thread discussing the set up process on linux.
It's not a walk in the park but it is cheap.
https://web.archive.org/web/20100130041340/http://www.dealextreme.com/forums/Default.dx/sku.12057~threadid.88670
