He's had fitness questions and is probably third choice for his position so I don't think we're in much of a position to play hardball here. It's been so long since he's been a first team regular that you'd maybe question if he still had it in him.
Clubs towards the bottom still aren't that rich. £10m maybe is fair, but you can't expect teams down there to be spending £15m+ on fullbacks.
The market for good players is inflated because teams want the best or the most potential. Nobody will be going above and beyond to sign Kieran Gibbs.