Messi is a marketing dream. He will make City more popular both in England and in the world. Therefore, he's easily worth 100m. The fans of the other top clubs in England will be quite envious, no doubt about it.
I don't think Messi will (significantly) improve us as a team though. City 17-19 won more points and had better GD than Barca's legendary team (09-11). And Messi hasn't been the known difference maker in the CL since 2015. We will probably create more chances with him but won't be sufficiently stable defensively due to a reduced ability to press and win the ball back against the top teams. If we allow Liverpool and Manure to play through our press, Koulibaly won't save our asses.
Pep will probably try to fix that problem by using press machines like Jesus and Foden/Bernardo. Chances are that Messi will want to play with Aguero though. So, it might not be easy for Pep.
Give Barca Garcia, Mahrez and/or money but not Bernardo or Jesus. That would be a mistake, imo.