As usual, google is not your friend in terms of finding the amount of ink in an inkjet cartridge. Nor does there seem to be ISO standards yet on 4x6 photoprinters. All I can seem to find in a quick search is the #33 standard yield cartridge and the #35 high yield cartridge at $27 and $35 respectively. And because its only a 3 color cyan, magenta , and yellow printer, which will mix all three colors to get a muddy looking black, in my mind, the P315 is not a true photoprinter.
Not sure, but some third party Lexmark refill site seem to list a 4x6 page yield at up to 190 prints for a #33 OEM, or 14.2 cents per print ink  consumables minus paper.