Buying Griezmann for £60M isnt buying at the "cusp".
If we pay a transfer record, I'd expect the player bought, to be at his peak.
As an example, Barca got Suarez for a huge sum of money, when he was at his absolute peak. They were buying a player of the highest quality.
We got Di Maria for a huge sum, when he had just had his best season (again, at his absolute peak). MUFC supporters loved this guy when he was here, but turned on him when he went to PSG. He is a player of the highest quality, who can win games for his team.
For an upcoming player, I'd expect somewhere in the region of £30M, which is what we paid for Depay.
The other problem with upcoming players is that they have considerable risk. You could get a gem like Ronaldo or Hazard, who become great. Or you could end up with someone like Anderson or Andy Carroll, who fizzle out into nothingness.
If we cant get someone at their peak (and pay £60M+), then I'd go for someone who is past their best, for a cheap price, high wages, but a short contract.
Examples of these types of players are Bastian, Larent Blanc, Henrik Larson, Sherringham.