Isn't that atypical of incremental pricing? One would expect the marginal cost of the third day to be lower, if anything.

We organise a smaller (a lot smaller!) conference¹ in Helsinki, Finland and I can tell you that a surprising number of things increase in cost linearly as you add more days.

Of course adding one more day to a two day conference doesn't double your costs, but I'd image that Google is taking into account inflation as well as just trying to increase their profit margin (though taking into account the amount of hardware they give out they give out to attendees does indicate that they're also trying to limit the audience to actual developers).

¹ http://2012.frozenrails.eu/