shouldn't increase the price of pdfs - and digital printing doesn't change in cost greyscale / colour as far as I know? Depends what kind of size of print runs you do (i.e. if Litho, then yes, big difference in cost between monochrome / colour), but something to think about?
it is a little bit of a let-down, visually, to go to b/w text pages after those brilliant covers, and there aren't many rule-sets these days that aren't in colour.
Not having a go - just (possibly) constructive (if admittedly superficial) feedback!