Last Christmas, we tried to park in the economy lot and it was full. Thankfully, there were people standing at the entrance handing out orange tickets that allowed customers that are being turned away to park in the daily garage for the same price as the economy lot!

So we saved $6/day and got to park really close to the terminal.
They do have a phone number now that you can call to check the vacancy status of the parking lots (703.417.7275). I find that DCA's economy lot shuttle to the terminal is much faster than IAD's. Getting from the terminal to the economy lot takes much longer at DCA.