The really, really big ones are useful even for pushing limits, such
as cr1.8xlarge, with 32 CPUs and 244GiB memory. Current spot instance
price (the heavily discounted "can die at any time" one) is $0.343/hr.
Otherwise, it's 3.500/hr.
Just to keep in mind cpus are similar throttled:
One EC2 Compute Unit provides the equivalent CPU capacity of a 1.0-1.2 GHz 2007 Opteron or 2007 Xeon processor. This is also the equivalent to an early-2006 1.7 GHz Xeon processor referenced in our original documentation.
Who knows what that does to memory bandwidth / context switches etc.
Jesper