Just price it right. Honestly ask yourself what amount of money would make you want to do it. That's what you nicely tell them it will cost.

I've done this and been surprised that sometimes people say yes. Just make sure you will really be happy at the price you quote.

If they won't pay it, you've either avoided burning a bridge or at least sent a signal that you are worth way more than they think. It can increase their respect for you in the long run.

Also, it is good practice for all price negotiations and the experience can benefit you in your other pricing and engagement negotiations.