If you regularly charge $2.50 for a cupcake with a swirl on top, then you would charge $375 to $437.50 for the order. If your cupcake has more complicated decorating, then the price will go up. Whatever you charge for your cupcakes, just do the mulitplication. Just because it's a large order, don't think you have to discount it in some way, unless this is a guaranteed repeating order over a period of time.
If you know the cost for supplies for you would be $50 for that order (as an example), then deduct that from the overall price and charge the difference.
Some people think that they'll save money if they buy the ingredients for the cake/cupcakes. However, are they buying the same quality ingredient that you would buy? And if not, is it going to compromise your product and possible your reputation.
Personally, unless it was a family member, I wouldn't be allowing a customer to provide any supplies. If they want my product, then they buy my product..