If you are flying one of the poor cousin airlines (i.e.: not NW, CO, or DL), the Big Blue Deck is covered, walkable (mostly), and about $6 per day. There might be other sites that are cheaper, but not enough to make up for the fact that they are uncovered.
If you are flying NW, CO, or DL through the worldgateway, the next-to-terminal deck has gone up to something close to $15 per day. I don't remember the exact amount, but it seems to go up every time I park there!