I say "market" because no-one really knows how the spot market place actually works. We've had machines run for weeks, and other times the prices fluctuate in bizarre ways and we can't get out preferred instance types for hours or (in the worst case) days. There's an interesting analysis here: http://santtu.iki.fi/2014/03/20/ec2-spot-market/
I looked through the pricing table and played with the calculator, it seems something equivalent to our needs would cost around a third more on google but each cpu would have twice as much ram. Not worth it for us.
Note: we're very aware of this pain point, and maybe you'll see something soon ;).
But to be honest about the situation, the cost would have to be much lower to make it worthwhile for me to rewrite our scheduler on Google's API.
EC2's spot market fluctuates based on supply and demand, and there's no reason to think the same forces won't apply to GCE.
disclaimer: I work at Google on Google Cloud Platform
California tends to be more expensive than other AWS regions, though (I can't remember the reason - perhaps just availability?). If you're in us-east-1 the price sticks around $0.20 per hour, and eu-west-1 is rock-solid at $0.32 per hour.
If you have work that can be done on spots it can be a good idea to make it region-agnostic so that you can take advantage of better prices for different instances in different regions.
For example, with Google you don't need to get a specialized instance type to use it's monster-fast Local SSD - just use the same instance types. This alone should simplify use of Preemptible VMs.