I personally find fleet commonality costs much harder than in previous games.
There is a hefty an upcharge for each additional aicraft type which goes for all your existing types - which does not make much sense for me... Would technicians start costing more (get a payrise? ) just because their airline bought a new type of aircraft?
You need to 1.) have enough technicians for each fleét type
2.) train the new types to the engineers (and with expertise, comes a higher level of knowledge -> higher salary)
3.) buy lots more of each part (with all-airbus, you only need some 5 spareparts of a kind, but with 3 different models you need 3*5 each part = 15 each part).
Just to name a few things...