I've seen a lot of him over the years at Utd, Watford and Forest and a fee of below £20mil doesn't surprise me. Perhaps £14mil up front with potentially rising to £20mil if he is a success and starts loads of games for them across his contract would be alright. But he's simply not that good. Games pass him by far too easily for a central midfielder. Include a buy-back clause if you can, because you might as well, but it won't get used.
He was decent at Forest but not one of their stand-out players. There's a reason why a number of PL clubs were clamouring to sign Djed Spence after his time there but not many are bothered about Garner. Even Forest haven't seemed that bothered about trying to sign him again. Remarkably the mooted fee for Garner still seems similar to Spence despite the latter appearing a better prospect. Garner hasn't impressed as much as the likes of Gallagher had in their Championship loans either.
He'll never usurp McTominay from the Utd side, however much many may wish for it to be the case. His set-piece delivery is fine but not fantastic - he's not got a James Ward-Prowse level of talent for it where you can afford for him to be a bit of a passenger at times in open play because of the clear added value his set-pieces bring.
Even within Utd's academy Mainoo and Iqbal will soon overtake him in the CM pecking order as they seem better prospects.