Under the standard contract, the builder is entitled to charge for his work in dealing with the extras like this, and if your contract is silent on the matter then I think the standard contract is a reasonable place to start.
There are two matters he is entitled to charge for, his additional work, and a profit element.
If he agreed an additional price that should cover his costs, and in fairness he should have explained at that stage his entitlement to add a profit element. If you are not in the trade, how would you know.
The traditional profit element is 5% of the total not 10%. Its calculated like this cost €2,550 divided by 19 multiplied by 20 gives €2,684 so his profit is €134 on the total €2,684.
I would push back fairly strongly, if he wanted a profit element he should have made that clear up front, 10% on top is an unreasonable profit element.