Right now I don't think constitution does enough to be worth buying. It only adds a couple HP, with no particular scaling. Considering what the other stats do and how hard units hit later in the game, it's pretty insignificant. This also contributes to the problem of champions being fragile in late game, because even if you dump five levels of points into Constitution, you're still not clear of being one shotted by a squad with maces.
So, a couple of suggestions:
1. Change the HP formula such that it's something like 10 + (Level * Con / 5). That'd mean every level with 10 CON you'd gain 2 HP, even if you don't buy more CON. It'd make more CON more valuable due to it contributing more as you get stronger.
2. Instead of that, change CON to be a scaling factor like STR and DEX are on Attack/Defense respectively, only for Health. Then you can add health as a stat to some items, like defensive gear. So a breastplate could give 3 DEF and 2 HP, for example. Since they're both survivability stats it makes sense, and it means the item still helps you even if you happen to roll a 0 on your defense roll.
Either suggestion would help keep champions competitive in the late game, where they tend to fall behind right now because they're so fragile compared to squads.