He possibly overpaid some, but Raji probably could have picked up an offer for a good bit above vet minimum (2-3 M) at some point. By signing him when he did, TT could worry about other limits in the roster come draft day. Without Raji, the only NT is a Viking castoff. 4M vs 3 or 2.5 won't kill the Packers or prevent them from re-signing the critical other players.
Vastly overpaying Raji would be paying him what he was offered before last year.