Tell them you have a google-like pricing model. Pay per Click, to the highest bidder.
I told someone the other day 40% surcharge for right clicks, 60% surcharge for shortcut keys and custom made actions/scripts.
Their every 5 minute emails came to a stop, and the next morning, I had a firm, well thought out list of changes, complete with an apology.
I am however, waiting for the job to be completed and hear from them re the shade of frosting of the vinyl being a different shade to what was on the photoshop mock up...
and yeah... good luck with your experimental carpet job... sounds fun.