An accurate way to determine ink cost is measure by weight in grams.
First weigh a full container of the ink to be tested on a gram scale. Take the price paid for that container and divide it by its gram weight. This gives you your ink cost per gram. Then take two blanks, print one of them and run both blanks through the dryer. Weigh both pieces on a gram scale to determine how much ink is on the garment. If you're really critical, you'll also need to take into account the ink left on the screen, squeegee, etc after the job is complete. You can also weigh these items and divide that into the number of pieces printed times your gram cost.
Then take two blanks, print one of them and run both blanks through the dryer. Weigh both pieces on a gram scale to determine how much ink is on the garment.
Which won't take into account any ink wastage, or any moisture evaporation from the ink, or any manufacturing variances between the printed and the unprinted shirt.
I know when I print a test print and attempt to estimate ink usage from that I tend to be way over compared to an actual print run. Personally I think the only reliable way to measure is a thorough before and after weighing of ink across a large run.
screen printing white ink cost 
