How to jokingly express an "if you pay me, I'll say it" attitude?

You can consider the proverb He who pays the piper calls the tune.

The person who provides the money for something has the right to determine how it’s spent. [OD]

My opinion on that is negotiable.

I'm an ethical man. I follow the golden rule. The one with the gold makes the rules.

You wanted something humorous and this certainly fits a classic pattern. You want something joking, and this is definitely going to be taken in jest. It's also something that gives you a way to back out if the first line gives you an indication that your audience might not be primed for such a joke.

The quote of the comedian Groucho Marx

Those are my principles, and if you don't like them... well, I have others.

could give the desired result in the appropiate context.