That will partly depend on the colors in your designs as the black carts cost a more than the color carts.
In the end, I decided to trap the cost in the month I expense them. Kind of like how do you allocate for toilet paper usage expenses?
You 'could' try to figure out the cost by noting when you started with all fresh carts, noting the number of shirts you were able to produce, and then dividing that out.
That didn't really work for me as my prints tended to be heavy in magenta, and I was flying thru magenta carts about 3 to one, but none were running out at the same time.
My thinking? I'll spend more time trying to figure out the per print cost that I didn't have time to afford for for that. I have seen people estimate 50 cents per print.
Not to worry, this is a common question. At some point, I think most of us wonder it, I did as well, attempted to find some formula with any accuracy I felt I could count on and did not find one (I also have a strong accounting background). I let it go. I believe, right or wrong, I generally allocate about 50 cents.
The allocation of cost greatly reduces, though, when you go to refillable carts. Epson Durabrite ink is expensive stuff! Best wishes.
Cost per print with Durabrite ink 
