So where I am we don’t use blown insulation like this, so please take this with a pinch of salt, but…
I redid 50m^2 of roof insulation in my house to exceed the insulation standards in my area and the cost of all the (very high spec) insulation sheets and materials was just over £3000. I don’t know about loose insulation that gets blown like in your picture, but I just wanted to contextualise why I thought the price seemed high.
If it was me, I’d set up a couple of cameras and mouse traps to see if you get anything. If not, add that company to your big book of Bullshit and chalk this down to a good lesson learned. Owning a home can be stressful and expensive enough without scamming dingbats making things more difficult.
If you do end up with mice in the traps, it would be worth getting a quote from another company, juuuust in case.