It looks like preemptable GPUs are exactly half the price of normal GPUs (for both K80s and P100s; $0.73/hr and $0.22/hr respectively), so they're about double the cost efficiency (when factoring in the cost of the base preemptable instance), which would put them squarely ahead of CPUs in all cases. (and since the CPU instances used here were also preemptable, it's apples-to-apples)
