Using transfermarkt.com valuations, comparing players he has signed only:
2010-11
Storari €4.5m (€0.5m)
Bonucci €15.5m (€20m)
Motta €5m (€1m)
Krasić €15m (sold €7m)
Martínez €12m (€0)
Pepe €10.1m (€3.5m)
Quagliarella €15m (€9m)
Toni €0 (sold €0)
Barzagli €0.3m (€10m)
Matri €17.5m (sold €12m)
Total: -€32.2m
2011-12
Lichtsteiner €10m (€17m)
Ziegler €0 (€3.5m)
Appelt €2m (€1m)
Almiron €0 (sold €0.4m)
Ekdal½ €1.31m (sold €1.5m)
Elia €9m (sold €5.5m)
Giaccherini €7.25m (sold €7.5m)
Pazienza €0 (sold €0.5m)
Pirlo €0 (€7.5m)
Vidal €10.5m (€44m)
Vucinic €15m (€15m)
Cáceres €9.5m (€9m)
Bouy €0.45m (€0.7m)
Padoin €5m (€3.5m)
Rossi½ €1.7m (€1m)
Taïder½ €2.425m (sold €2.5m)
Total: €45.965m
2012-13
Anacoura½ €0.75m (€0.012m)
Leali €3.8m (€5.5m)
Rubinho €0 (€0.3m)
Lúcio €0 (sold €0)
Masi €2.1m (€0.9m)
Asamoah €18m (€20m)
Kabashi €0.85m (€1.8m)
Isla½ €9.4m (€5.25m)
Pogba €0 (€34m)
Troisi €0 (€1.2m)
Boakye½ €4m (€1.5m)
Gabbiadini½ €5.5m (€4.75m)
Giovinco½ €11m (€7m)
Gallinetta½ €1m (€0.063m)
Buchel½ €1.5m (€0.15m)
Cavion €1m (€0.075m)
Anelka €0 (sold €0)
Total: €23.6m
2013-14
Ogbonna €13m (€13m)
Llorente €0 (€20m)
Tevez €9m (€22m)
Barlocco½ €1.25m (€0.1m)
Peluso €4.8m (€4.5m)
Berardi½ €4.5m (€5m)
Zaza €3.5m (€5m)
Total: €38.55m
Grand total: €75.9m
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You could add or remove bits here and there. For example transfermarkt are particularly low with current youth player valuations. Also I've not taken into account bonuses which will probably be triggered (Vidal, Tevez, Ogbonna). Additionally we could surely sell Vidal and Pogba for more than their valuations. Finally I think some of their estimations for more experienced players are above 'real world' valuations on the market, but ok in the books.