I don't do assignments or shoot for payment. I only sell my photography for stock or as fine art prints. But It's common practice to decide on the payment in advance before even shooting, and get paid enough in advance to cover expenses. Any portrait or wedding photographer will tell you that. Your price can either be a package price based on the shoot and a fixed number of prints ordered or a fixed shooting fee with a per print cost afterwards. But it's a contract that needs to be decided in advance so there are no questions or hard feelings afterwards. You also need to know in advance what your costs are going to be so you can give the client an accurate proposal.

Ditto, larry's comment.
I find it almost unbelievable that some one would buy something and NOT know the price beforehand.
Can we swap customers?

Ditto with previous responses. You may also want to consider preparing a break-even analysis, i.e. what it takes to recover your costs to break-even. Be sure to include not only your fixed costs but your best estimate of any variable costs. Once completed you'll be in a good position to know what profit you are willing to accept for an assignment. Finally, outline a business plan. Your business plan will help you focus on what you do best and not get into trouble straying too far afield.

Take care and have fun!