Ink Loss During Black Swap in Epson 4800 Printers

A couple weeks ago I made 75 Black-and-White prints, !5" square on 17x22" Espson Premium Luster Paper, on a friend's Epson 4800 printer. Now we're trying to figure out how much I need to reimburse her for the ink I used.

I've read several of your older posts, regarding the cost of making various kinds and sizes of prints on Epson 3800 and 4800 printers. The calculations I've made from your input brings me to between $2.15 and $2.75 per 225 sq inch print. (You-all seemed to be talking about color prints, however; do black-and-white prints vary from those figures you proposed?) Do these figures seem correct?

Mostly, I'm unclear about the amount of ink lost during changeover from Matte to Photo Black ink. After I was finished doing my Photo Black prints, we changed back again to the Matte Black cartridge. (One commentator referred to "round trip" swapping; is that what they referred to?) Any ideas how much ink is lost during that swap, and then the second swap back?

Are there any variables I'm not considering? Does "charging the ink" come into play during these swaps?

My math tells me that, when one buys 220 ml cartridges, they're paying (here in LA) about 50 cents per ml. Does that ring a bell?

Thanks for your insights!

THOMAS ALLEMAN