Yesterday IVR cost only three dollars but today it rose to five. Why?

“Yesterday we agreed on the cost of IVR message but today when I sent you an order the prices has increased three times though I added just a few files. What can I do if I have already agreed the price with my boss? Does it matter much to the voice talent if he/she is to read some 60 words more?”
I often feel uncomfortable listening to similar statements in the negotiations with customers. Our price list is given in our website and is available to anyone. If you have any questions you can always obtain more information. Of course, this service has a specific character which makes it difficult to grasp. That is why let me explain. Suppose you need to have a tooth filled. You make an appointment with a dentist and specify the price. But the next day you find three more holes which also need filling and you ask the dentist to fill hem as well while you are in this comfortable chair but at the price agreed before. What do you think the answer will be?
I am sorry if my explanation sounds a bit tough but at any rate I have
made myself understandable.