Finally, managed to cap a stat:
I'm lvl 20 (and I removed any item with how to on the screen), so I have +190. It may be 1+2+3+4+5+...+(Lvl-1) = Maximum. We will know once I lvl up once more (but i'm farming DL) or if someone manage to cap with a different level (I tried with lvl 2/3/4/8 and I have the same result, so I think it is). The cost was over 9000 for the last point (Litteraly).