I also would prefer an Art/Lore book with the art being used as reference for paint schemes and the lore so that I can throw out interesting facts whilst playing the game but ultimately it would come down to cost
Yes I guess you're right.
But if Monolith already has the print files .. if adding some content would result in, say, 40% more sales? A few more pages may indeed get a bit more expensive but on the other hand, one would expect the price per copy to go down. And the rules aren't full-colour pictures so less pricey per page.
So basically it comes down to, what do we want in the artbook?
e) paint guide
... anything else? :-)