Another way you can look at this is, you being new to the business neighborhood and wanting to make a positive
impression, that trying to accommodate them can be a wise and ultimately profitable investment.
The trick is not to get yourself locked in to a low price, now and foreverrrrrr morrrre.
I would start by working the numbers to determine a reasonable charge for exactly what they want. Depending on
how much past what they want to PAY that figure is, you can then write it up that way, and then tell them you're
willing to discount them down to the 500, (or however less is reasonable and you're willing to do) this occasion
only, in the interest of helping each other out.
Now, whether you CAN do this for 500, depends a lot on what they want. You didn't say how many appetizers,
but you can certainly make some cheap appetizers and meats for 75 people with that budget, and most
definitely if you're willing to forego some profit.