The answer to this is the same as for the question about how much do full C cup implants cost.
The cost of a breast augmentation using breast implants is one thing for the surgical fee to carry it out, one thing for the facility fee to carry it out (which may or may not include professional fees for anesthesia or lab tests), and one thing for the implants.
Different plastic surgeons and facilities handle this differently but all costs should be spelled out to the patient before the surgery for elective cosmetic procedures like breast augmentation.
Most plastic surgeons I know view the cost of the implants as a "pass-through" expense and charge the patient what the surgeon or facility pays for the implants plus perhaps a "shipping and handling" charge. Saline-filled breast implants cost one thing (depending on the manufacturer) regardless of the size and silicone gel-filled breast implants cost over twice as much regardless of the size (again depending on the manufacturer). Therefore, a breast implant should cost what it costs the surgeon and is charged by the surgeon or surgical facility regardless of its size. The major difference is in whether it is a saline-filled type implant or a gel-filled type implant.
It is also important to point out that there are no "B cup" or "C cup" size breast implants. Breast implants come in a variety of dimensions and volumes just as dresses or shoes do and what would augment one woman's breasts to a C cup could be completely different for another women just as their shoe or dress sizes could vary but still fit or have the same effect.
In my opinion and experience, the proper way to size breast implants is to know what size breast the woman is starting from relative to her chest circumference, know what the width of the breast is as this determines the diameter of the implant, and then gauge the effect of the forward volume of the implant depending on whether the patient chooses a low, medium, or high profile implant (with the diameter held constant). A low profile implant should increase the cup size by one, a medium profile by one and one-half, and a high profile implant by two cup sizes relative to the same chest around and relative to what size the breast was to start. This is far from exact and of course there are exceptions but it works much better than any other method used to determine breast size results.
