Monster has a flat wire that's reasonably priced, but if you check your local Radio Shack or Home Depot, there may be something even cheaper...Monster wires and cables are about 30% more expensive than comparable brands.
Something else you might consider is getting dirt-cheap wire and running it through a length of plastic tubing on the wall, then painting over the tube. Home Depot carries these tubes, which include an adhesive backing, specifically for wiring purposes such as speaker mounting. Again, you're looking at just a few bucks, and once painted over they blend in nicely with walls.
