I regularly get trousers turned up by a professional service and about 10-12E is about right.
If you consider the minimum wage, and that it is an hours work to turn up a pair of trousers, then you would not expect to pay any less, especially for someone skilled to do a good job.
I turn up jeans myself, if I am not too bothered how they look and the ones done professionaly always look better. If the trousers or jeans have cost you a bit of money an extra tenner to make them look good whenn turned up is worth it.