Does the weapon influence the usual amount a VII skill would cost? Like for example if a Greatbow had a VII Rainbow Peacock?

Individual rank 7 skills aren't worth anything. You can ID them or skill change for them and there is actually an OVERSUPPLY of regular skill set cards (1-5 and III, IV, V, and VI) due to the fact they are so common.

On a side note... please, try not to double post. You posted in both the General Discussion thread as well as the Guides and Strategy thread.