I can't remember exactly if it's the grey you really need or it's the blue that is also referred as grey.
Anyway, on the advice of Fergiebilly I bought the grey stone and I'm very happy with. As to what you should expect to pay for one in good condition then my guess is around the £80 mark.
I tried to get a bigger stone a while ago but those are well over £200.... I'm happy using shavettes nowadays....zero maintenance
The small Thuringians in the box you can find for around 100€, sometimes cheaper if you are vigilant. They don’t have the Escher label. There is an ES in the middle where the emblem is. The rest are just Thuringians and not Escher. The big ones go for anywhere north of 300-400€. Couple of yellow/green sold not too long ago for 2000$+ each recently.