This is a great question and one that I wrestled with myself.

First, dont listen to the bike snobs... A good touring bike can be bought for less than $400. Here is my suggestion: Go on craigslist and look for 80s and early 90s road bikes. Typically you would want to look for a bike with a cro-moly steel frame. Cro-moly is what most bikes were made of back then and is the perfect mixture of strength and weight for touring. Ideally, you would want a bike with eyelets for your front and rear racks- these are basically holes where you can screw the racks into the frame of the bike, they are not essential, but they certainly make the racks easier to attach and stronger. Lastly, depending on how much gear you are carrying and the elevation changes you will be doing, you may want to have a triple crank (three cogs in the front). This will provide you with easier gears for carrying heavy loads up a long climb. Other helpful things include the braze-ons for water bottle cages and a decent set of tires, but you can always add the tires later or make other arrangements for your water bottles.

Here are a few older models that I regularly see on craigslist for under $400 in great condition that are excellent touring bikes:

Schwinn Voyageur
Schwinn Voyageur SP
Trek 520
Trek 720
Miyata 1000
Miyata 610
Fuji Touring Series

If you go this route, you will need to learn to friction shift... but its easy, fun, and mechanically more reliable and simple. Friction shifting for life!