Building up a frame will certainly be more expensive than a built up bike. I am in the process of finishing up a Nashbar Touring frame build, and it looks like I will be over $750 before I am done. Last spring I picked up a Fuji Touring bike (same as the Windsor Tourist) on closeout that came to under $600 with tax. I had trouble with the wheels and ended up returning that bike, but on reflection I would have been better off keeping it and just swapping out to a better set of wheels. The XT hub/Mavic A119 set that I have on the Nashbar cost me under $200 and are hand built and seem to be rock solid.

If I were in your position I would look for a good used touring or sport touring bike, learn to do some wrenching on it and decide what features are really important to you. For commuter dress a set of fenders, rear rack and cheap set of panniers and you are good to go. You should be able to put such a package together for a couple hundred dollars.