Yeah, when I started actually trying to back it up, I noticed I was probably a factor 5 off.
Namely, if you look at the keylength.com values for asymmetric key sizes, 768 in 2009 ago should come close to the difficulty of 896 today. The RSA 768 challenge was broken in 2009 (http://eprint.iacr.org/2010/006), which cost them "the equivalent of almost 2000 years of computing on a single core 2.2GHz AMD Opteron". Renting that amount of time Amazon EC2's $0.06/hour instances would be $1 million.
I'm not sure how they compare in practice, but it might be worth calculating how many hours an Amazon G2 instance would take, using their high-end graphics cards as CUDA processors. I think the cost per performance ratio is much lower, and that could change the equation in the other direction.
Fair enough. I've read that using CUDA (or another GPU-based language) you can get at least a 10x the GFLOPS of a 4-CPU Xeon [1], though, and RSA cracking should easily parallelize, if I'm understanding the process correctly. And the high-end NVidia cards in the G2 instances have 1,536 CUDA cores each. No, I'm not kidding. The one benchmarked in the link above is about 1/3 the GFLOPS of the one in the G2 instances.
And it looks like a reserved G2 instance is 0.65/hour (though can be lower on the spot market and in the reserved instance marketplace). So if there's a 120x speed improvement over the "single core 2.2GHz AMD Opteron" (and that's assuming each core is as fast as the Xeon core above), for only 11x the cost...well, it gets a lot cheaper.
In fact, it ends up, if I haven't done my math wrong, at about $94,900 of full instance time (less if you get spot or reserved instances). [2] To win the $200k prize. Hmm....
[2] "the equivalent of almost 2000 years of computing on a single core 2.2GHz AMD Opteron": That's 17,520,000 hours. If the G2 instance gets you 120x performance improvement, that's 146,000 hours. At 0.65/hour, that's $94,900.
Namely, if you look at the keylength.com values for asymmetric key sizes, 768 in 2009 ago should come close to the difficulty of 896 today. The RSA 768 challenge was broken in 2009 (http://eprint.iacr.org/2010/006), which cost them "the equivalent of almost 2000 years of computing on a single core 2.2GHz AMD Opteron". Renting that amount of time Amazon EC2's $0.06/hour instances would be $1 million.