I agree more with this reply than any of the others...increase, not merely sit on the original quote! Show the true value you place in your work/services!
In addition to the aggravation (or more likely, insult) they've caused, you could add the usual increases in on-costs that have occurred since the original quote. Fuel, electricity, materials! It should finally get the point across to them that you aren't doing it for their pleasure alone, but as a business. Hopefully they'll withdraw from the negotiations and find someone else, who'll do one of two things: take it on for their price and get shafted in the process (no longer your problem), or come up with a quote well above yours which will prove your worth.
Anyway, one thing I've learned from taking on public art commissions, always add a buffer to the quote!:rolleyes: It can then work as something for negotiation, or more likely as a contingency, for when things go pear-shaped (delays, increases in materials above original quotes, reworking parts for the fluff-up fairy:-).
As for David's problem with catering, I can't believe someone would baulk at that! Even a basic buffet for a wedding costs at least $35 per head. You couldn't get a fried food meal for a poor student at the Refectory here for $6. Again, quote The Castle's best line...
Cheers