Compare the price of a new bumper cover with an OEM Ford bumber cover. If you decide to go with a bumper cover from a junk yard, you should not pay any more than 50% of what you would pay for a new part.
I know you said that color does not mater, however remember that you might get lucky, and find a bumper cover that is exactly the color you want, and that could also save you some money in painting costs.
...Richard