Penso que seja isto:
"The average cost is the total cost divided by the number of units produced.It is important to understand that firms maximize profits by considering the marginal cost, not the average cost. The difference between the average cost and the sales price does determine the profits per unit once the profit maximizing quantity is determined, but the profit maximizing quantity generally does not maximize profits per unit."

In other words, according to some formula they have, they have 'weighted' the cost.

The mean weighted cost of double NRTI therapy dropped from US$ 3,810 per patient/year in 1996 to US$ 630 in 2001 (preliminary data), with a 84% reduction.

The reduction of the mean weighted cost of triple regimens including PIs or NNRTIs is estimated at 57% and 66%, respectively.
http://www.google.es/search?q=cache:8GB8uihLAiIJ:www.aids.go...

The text in fact refers to Brazil: http://www.aids.gov.br/final/biblioteca/drug/drug6.htm




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Note added at 9 mins (2004-04-26 11:40:07 GMT)
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Below is the average weighted cost per tablet of oral triptans in DoD medical treatment facilities (MTFs). The new contract price for zolmitriptan is $3.20 per tablet. This represents a 55% savings over our most used and former BCF oral triptan, sumatriptan.

http://www.google.es/search?q=cache:6qyDNyv74q8J:www.pec.ha....

Revised Cost