The only thing he has done wrong is not tell you upfront that his price does not include adhesive and grout (or maybe he did but you miss-understood) and he should of put it in writting. As for you working out how much adhesive he needed it is very very unlikely that your estimation of it would be correct. I have never tiled a wall or floor that is flat unless i have done the preporation work myself.
We don't know the full details but do take into account all the things that Sean mentioned, plus it's his knowledge and labor and skill that you are paying for........ We are all entitled to earn a fair living.
Are you happy with his work?