You can't really generalize about per-installation costs. Digging fiber out to several houses is much more expensive than digging over to a 200-apartment complex, which lets you split the bill for laying fiber between 200 people.

In Copenhagen, Denmark, my apartment complex just got connected for 3800 DKK per apartment. And that's in a country with fairly high wages and 25% VAT. For similar apartment complexes (large ones, close to each other), in the US (where labor is cheaper and there isn't 25% VAT), I imagine Google can easily do less $500 per apartment, and possibly reach $300.

Provided Google doesn't have to pay a tonne in fees to get to lay fiber in the ground (which is probably why it's only available in select locations).

There are really two costs for installing fiber. The cost to drag it past your house, which is expensive, probably over 1000. And the cost to bring it from your curb to your house. I've heard 500 for that, but it could around 300. Or google could just think of it as an advertising cost.