I fixed it myself mainly because the "repair centers" around here charge as much as the phone cost for any kind of service, and they usually do a crappy job doing it.
Second i don't like throwing a gadget away just because ONE button is not working anymore.
The repair got through quite smoothly, a bit of trouble unscrewing the lock button but everything went well in the end. Nevertheless it seems like the button was not the problem but the power sensor that is pressed by the button.
Better make sure to have an accurate diagnose before buying spare parts.