Great step-by-step instructions on building a raised bed
I have found a very nice set of instructions that shows how to build an 8x4 raised bed -- see Sunset magazine online, link posted below.
I should note that they estimate costs at $187, but that includes cedar wood as well as soil to fill the bed. So for the bed itself, using more standard lumber, would be a fraction of that.
It has some nifty suggestions, such as using tube straps to attach PVC to hold a netting or shade frame, if you wish to attach one.
I have not made mine yet, but I think this is the model that I will use.
Here is a link that might be useful: http://www.sunset.com/garden/perfect-raised-bed-00400000039550/