Wasn't sure how to approach gauging that issue, so i did a bit of math - the example i gave would normally be 300+180=480 points, but adding this joker increased it to 945, which is about double, so effectively a 2x multiplier for 2 hands. If we added a third hand, another 75x4, we get 780 points without this joker, and 210x11=2310 with it, effectively a 3x multiplier. I think this ends up being roughly an Xmult equal to Hands used in a round.
This isn't completely broken, but does need some tuning. So maybe just completely rework the effect to: X1 Mult, +X1 Mult for each hand used this round. This is a little over half effectiveness, and only gets high Xmult the more hands they use in a round, which means losing some hand money. If there are more ways to gain hands in the full game, maybe it makes more sense to be +X0.75 or +X0.5.