Firms differ, and some may negotiate special rates for major projects, rather than charging posted or even discounted bulk rates. Please cooperate fully. You should respect the consultant's right to do this, and an agreement should not be entered into if this is not possible. The consultant or engineer has an obligation to be totally objective, candid and honest with you. There are many valid, economic reasons for using consultants and consulting engineers.
Consultants come in many shapes and sizes and with varying levels of professionalism and capability.
Some consultants can fool some of the clients all of the time, and all of the clients some of the time. Clients sometimes fool consultants, too. Such behavior reflects unfavorably on both sides. Selection of the proper consultant, engineer or firm can result in extremely valuable service provided to a client needing assistance. However, if both sides do their homework properly, the results will be mutually beneficial, thus creating future opportunities for both.