You can find the official prescribing information for Velcade here:
http://www.accessdata.fda.gov/drugsatfda_docs/label/2012/021602s031s032lbl.pdfThis shows that the only form in which the drug is supplied is in a 3.5 mg single-use vial. It comes as a powder and must be mixed with 0.9% salt water to prepare it for injection. This results in much more than is needed for any one dose, and any that is not used within 8 hours must be discarded. So it seems that your provider is correct; the amount that must be purchased is the same for each dose, regardless of how much is used.
However, the calculation of how much of the cost is your share is another matter. You did not say what kind of insurance coverage you have, nor who told you the amount, nor how they calculated it. You should ask them to explain this in detail, and check what they say against the terms of your coverage. What you were told could easily be wrong.
If your share is difficult for you to pay, there are lots of programs to help cancer patients with such costs.
--Larry D'Addario
Pasadena, CA