The value potential of a sealed product varies wildly. It is so hard to pinpoint a set expected value and even if you did the standard deviation would be so large that opening a single box would be so hit or miss. Worst case scenario can be pretty bad and best case scenario for certain ones have gotten to where you won’t do a ton better than breakeven (as it should be for old scarce sealed product). I’ve opened some boxes that graded mostly 8’s pack fresh with a couple 9’s so you can price something like that out for worse case scenario. I’ve opened an aquapolis box that had no crystals and some boxes with only 10 or 11 holos. Then I’ve also opened multi charizard base set boxes where they both got 10’s and I had 13 holos total.
Looking at gym challenge specifically the box will set you back $2,000 plus grading costs. I would fully expect that you could lose half or slightly more unless you hit some of the key 10’s. Even 12 straight 10’s on the holos could net you a loss if they are all the less desirable ones as many of them go for ~$100 or even less. If you hit 6 10’s (as my previous work shows ~45% 10 rate)) and 6 9’s I would say that would put you right about 50% of the box price on average where the range may fall from $400-$1500 so again a huge variance.