That quote seems a bit high. I'd expect you to pay that if it went back to Tag. Alternatively, if you want something cheaper, find a good local watchmaker. I don't know where you are based, but if you go on the BHI website you can locate a good watchmaker in your area.
They sent my Tag away, certainly wasn't done in house. I know this because it took what felt like 50 years for them to return it, and a crappy job was done - the hallmarks of having Tag do it.
TBH I can't see Ernest Jones having an in house technician, or is that what the inverted commas were for. That quote might be for them to send it off to Tag, plus a bit of commission for themselves. As previous post says, seems a bit high otherwise.