Keep it cheap
not a lot of people put thought into this one, but I know for a fact the line are smaller on 8 pass then 4 pass, if the density and linerization where set correctly when setting up the ICC profiles on 4 and 8 pass the ink consuption would be the same. As for cost. Who knows, I'm sure HP has an RD dept. that knows that question, but what you said around 30 cents sq. ft. for solvent prints sounds right. Just make sure you get your ink's at a good price, I have a couple clients I supply ink to. $155 per cartridge. 1L Cartridge.
Hightec Digital
Large format printer technician on Mimaki, Mutoh, HP, Roland
916-990-9803