Hi CP,
Are you talking about real venetian or a drywall mud finish? I don't get a lot of oppourtunity to do a venetian type finish, I use drywall compound, but when I do I charge by the hour and not nearly enough as all my customers say I don't when they pay me.

I seem to forget that I'm not just painting and that there should be a charge for my talent as well as the hours put in.

I think you should charge enough to make you happy at the end of all your hard work but no so much that the client feels that you are taking advantage of them or that you feel that you've taken advantage.
Good luck
Mary