I know there are a 100 reasons or more an airplane has to make an unplanned landing..from a suspicious reading in a cockpit to a crack pot leaving a scary note in a lav. There are small diversions (eg I was on LH to GRU and due to fog we had to divert to Campinas for 3 hours- along with 10 other VLA's from UA to JJ) and then there is the entire fuel dump from a 777 or 747 or A319 and a return to the departure airport. I'd assume an entire fuel dump from a 747 would cost a lot more than E190! And many, many more reasons and circumstances and types of diversions and a wide range of different costs.
Naturally I assume none are cheap and I also figure they have planned somehow for these unavoidable expenses. Are they insured for those incidents to recover fuel costs and potentially even emergency vechicals, hotels, missed connections, landing charges? Can, after all these years a UA, AA or DL and others have an average forecast of about how many diversions may occur and accrue for it. But, it seems "insurable". Costly insurance, but insurable. Or do they really just take the hit on their bottom lines?