I've always felt that the club were slightly too generous when it came to renewing Messi's contracts, in the sense that he seems to get another one every 1.5 years, regardless of the length of the actual contract. I've no idea as to the actual current earnings, but he is obviously earning an obscene amount of money. On the other hand, as stated, if anyone "deserves" it, he is that person. The club over the last decade have absolutely hoovered up leagues and cups, the club as an entity has grown massively, the shirts are flying off the shelves, sponsorships, casual fans, etc. Messi is the fulcrum of all this success and growth. I'm the same as most normal punters, you think "if you are on 20M, do you really need 25M?" and so on, but obviously the players at that level get used to earning that kind of money and want more.
As an aside, anything resembling a "signing on fee" for a player already on your books is a bit of a joke. For any player.