Are you screening for M- units? is anti-M clinically significant or you always give M- units?
your cost per antigen is very good. If I am paying that much I may consider buying from my supplier. It all depends on antigen you are screening for.
eg. anti-S 1 drop of anti-S will cost you $15 (if you are paying $900 for 3ml vial), add controls, frequency of finding units, other supply cost plus tech time.
it would be a good exercise for you to take your Jkb example and add all costs to see what was your cost per unit and compare that to your supplier cost.
By the way your supplier is really giving you good pricing? east cost or west cost?