That logic is totally wrong. Argyle are paying him 2k (or whatever it is) a week because that’s all they can reasonably afford, not because they think he’s a League 1/2 player. All parties will be aware that Cooper’s long term future will be away from Home Park and that Argyle can’t remotely afford to pay 20k+ a week, so that sort of wage negotiation would be a non-starter and not even in the minds of the player and his agent.
By that logic, then Swansea were paying Flynn Downes c80k a week, as he’s just gone to West Ham for £12m. The bigger factor is the potential of Downes, the ability of West Ham to easily spend that sort of money and the fact that he had three years on his Swansea contract.
Premier League clubs are so vastly wealthy that £5-10m is nothing to them, and it’s especially nothing when they can get a future England keeper for that fee. That wealth is much more of a factor in big transfer fees than the Bosman ruling and current wages.