Honestly not sure. I have it upgraded enough to double my pay and I'm currently level 55. Doing a game of survivors nets me about 200 thousand, but my end of round pay is only 20 thousand.

No, it is not worth spending skill points on. This is especially true if you are F2P since you can't reset skills. It can be worthwhile in the beginning, but past around level 50 damage skills are far better to have.