Mancini bought Aguero, Silva and Yaya for a combined 80m in his time, in todays market these guys are going close to 80m on their own, if you include inflation, then yes in a 3 year cycle they probably spent more or less the same amount Pep spent, yet in that 3 year cycle only Pep is the one to win back to back league titles, so the question shouldn't be they could do it, but instead WHY they didn't do it? The lack of spending I'm sorry simply doesn't apply here both spent money in an era that the record fees were a lot less then Pep City era.
What different chellenge? When we came to City he had Hart, Kompany, Robinho, de Jong and Zabaleta. He further added Adebayor, Tevez, Lescott, Santa Cruz, Kolo and his second season he added Aguero, Dezko, Yaya, Silva, Balotelli, Kolarov to the squad this is good enough to win the league title back to back. In his final season he lost the league title to RVP.