It is valued on the ability of the player and the age of the player or other attributes such as injury prone etc, and seeing as Messi is still a 3ish years away from his peak ability he should be valued at about double ronaldo's transfer seeing as he is 3 times better than him and still at a younger age. When the transfer for ronaldo was complete he was 24, and atm Messi is 25 and he is such a better player than ronaldo was back then so if any team was to buy him they'd have to be loaded.