Not QM but Transfermarkt use market values, these aren't necessarily representative of real life values. It's more a metric based on contract, age and ability etc.
That doesn't make a lot of sense. It's precisely because the market value of a player like Branthwaite is so high that transfermarkt shouldn't be taken seriously, and certainly not used as the basis for a quiz. The lowest I can imagine Branthwaite being sold for in the summer, if there's only one offer and Everton are on the cusp of administration, is £60M. If Everton sort out their finances and there's a bidding war, he'll go for closer to £100M. And remember that the transfermarkt figures are in euro (1.17 to the pound at the time of writing). I doubt you'd get any change from £50M for Mainoo or Bradley either, although I don't imagine we'll find out any time soon.
I don't know of a better source for player values, feel free to suggest and maybe I'll change it :) Also transfermarkt is updated quite often so I think a player like Mainoo should skyrocket sooner than later