I think £60 odd million for Alisson is one thing, but the Kepa money is another thing entirely. Alisson has just played the whole WC for Brazil, keeping Ederson - one of the prem's best keepers - on the bench. Plus he's played in the CL for Roma. Kepa has got one international cap for Spain, spent the WC on the bench, and I believe has only played a few games in the Europa League.
They could both turn out to be two of the world's best goalkeepers but I would much rather have Alisson of the two. Chelsea are taking a big gamble on Kepa, they've really been forced into spending over the odds because of Courtois' refusal to stay and the lack of what they consider top level alternatives.