i had my old 106gti helicoiled in situ, think it ws about £50ish
they insert an airbag i think into the top of the cylinder and inflate it, then insert the new sleeve and thread and then blast all the s**t out with compressed air i think and then remove the airbag from the sylinder, plug back in and jobs a good un.
i cant be exact on the price as i had it done with some other work but i know i remember thinking it was good value for money.
tbh sounds like they have done it though and whoever did the plugs for the new owner as a favour has made a mistake and wont admit it and blames previous owner.