Well I had a nice screen shot to show, but realized that I would be displaying all my stats. (I realize i could have blocked them all out, but then it just looks silly and shopped).

If you hover over that Constitution tile, it will say:

"Constitution (Lv XX)
Gained + XXXX Hit Points"

Followed by the time, costs, requirement. It is my understanding that this is the exact amount that is suppose to be added to each heroes hit points.