Satnavsystems just completed my one on Friday after initially having problems with it. With app connect & voice control onto a factory Discover media unit and factory MFSW set up it was £389 inc vat.
I think, though not sure, their 459 quote to me included coming to me in notts to do it. Or maybe is was such a polava they put the price up. Eitherway, defo not worth it for that cost.