Well, you can buy a good one, or a cheap one hehehe...never the twain shall meet. I've got a Caldwell "The Rock", which uses the Protector style bags. Now, I've seen guys at my range use 5lb bags of rice as bench rests, and it does the trick. If, however, you want to get a better quality rest which will last a good long time, take a look at the Sinclairs, Harts, or Witchitas.
This is the one I think I'm gonna get...almost all of my shooting is done at the range from a bench, so I can justify spending more than I'd like.
[url]http://www.sinclairintl.com/cgi-bin/category.cgi?category=RIBBRICO&item=04-2725&type=store[/url]
I know that the rest I currently have is merely adequate; it seems as though I'm never really happy with it. Now, if money was no object, I'd get a Farley. A guy at my range who is into Bench Rest shooting has one...$650 worth of SERIOUS rest. It has a hydraulic handle on it for real-time 360 degree movement for fine tuning your aim after each shot...WAY bad.
If you think you're gonna be doing a lot of bench shooting in the years to come, get something good. If not, there's plenty of lower cost rests that will serve their purpose.