Per Epson's webinar details on the P900 ink, since you don't have to worry about the black switching, they made it a little smaller and it's also based on how a handful of users may not print as much and the cartridge would sit in the printer for a while. They did work on the cost of the ink so it will be less expensive and pretty close to the price of the P800 larger ink per cost area; basically less ink = less expensive.
IT Supplies
Not sure what you mean by "ink per cost area", as you do not explain that. In the Netherlands (and in the UK, see the other thread about the P900) though the cost per ml is NOT close to the price of the P800. Price for the P900 compared to the P800: €0,78/ml versus €0,64/ml incl VAT.
BTW: The costs per ml for the Canon Pro-1000 are €0,56 incl VAT (at the same supplier). That is a huge difference with the P900. Yes, the Canon uses a lot of ink for its maintenance routines. For the P900, time will tell.
"Less ink = less expensive". If you mean the out of pocket costs, you are correct. But the cost per cartridge is irrelevant IMHO. What counts are the costs per ml and these are significantly higher with the smaller cartridges.
The ink consumption will be a bit less with the P900 as there is no black ink switching (hurray!), but how much that is taken into account in your statement is not clear to me, and will differ of course a lot per user case.
cheers,
Remko