Mancini spent brilliantly at City imho, Hughes and Pellegrini terrible.
His first season was phenomenal in hindsight - Silva, Dzeko, Toure, Milner, Balotelli, Kolarov and Boateng (who we couldn't keep)
His second season was arguably as good with - Aguero, Nasri and Clichy (Nasri was good under Mancini) and scored the decisive goal in the Stevie G slip season after he left). Savic (who like Boateng was a good player who never quite felt comfortable and is currently doing well for Atletico.) Even Pantillimon did well for a £2m keeper.
Mancini's final season was his only poor one in the market and many believe those were Marwood signings, they certainly didn't fit the Mancini profile and as we know all was not well behind the scenes.
Sinclair, Rodwell, Javi Garcia, Nastasic and Maicon.
All in all I'd say Mancini's spending was excellent. What came before and after is where we spunked a boat load of money.
Hughes on the other hand spent £270m in 18 month around 2008 and £62m of that was on Adebayor, Jo and Roque Santa Cruz