When you buy a business you do so for the value that that business can generate for you, that doesn't always mean hard cash. The States bought Aurigny for the four pairs of Gatwick slots, and to protect the life line routes into the Guernsey.
Can you imagine how much more would have been spent (by the people and businesses of Guernsey) on air travel to/from Guernsey over the last five years if Aurigny were not there providing competition to flybe and Blue islands?
A lot more than the States has invested in Aurigny.