Funny how people don't understand the bumpers.
Last time I read about it, it was something like 2.5mph no damage to the car, 5mph no considerable (less than $1000) damage to the car.
The bumper skin isn't going to run you more than $1000 or so. Since you don't need to replace the foam, bumper beam, or mounting equipment (except maybe some plastic clips), you should be out with a ~$300 bumper skin and a couple of hundred to paint it. www.subaruparts.com
may have the skin for a lot less, I don't know what they run. Considering a complete bumper replacement usually runs in the ~$700 range, yours should be less.
It's probably just a freak coincidence that your bumper cracked. Combination of the angle that you hit and possibly some flaw in the plastic.