No, that's a good price. The ProMix bales are about equal to 2 of those, maybe a little more (hard to really estimate with "compressed" bales). I used to like the Miracle Gro mix but switched when they started doing the "moisture control" mix, which I just didn't have good results with.
I think a lot of the big nurseries make up their own potting mix, but it usually is similar to Pro Mix or Miracle Gro Mix. Sometimes you can get bulk potting mix at a good price if you chat up the nursery owner... ;-)
