Ronaldo had the ability to single handedly win games, even if the midfield was non existent. Pogba cannot do the same. He is a major presence in the middle of the park & a technically brilliant signing, but his value added is never in line with the same market value as the flamboyant wingers/strikers.
Its a bit like when Messi won the Ballon d'or (I forgot the year), and he shook his head during nominations and pointed to Iniesta as the main provider. Iniesta is/was a genius, but if you took his market value it wouldn't reach the same as Messi's.
Its the same when you compare two slightly lesser players in Pogba and Ronaldo in transfer valuations.