Keep in mind that with EC2 you're billed hourly so the fastest a 70 cpu-hour job could finish on m3.medium for $0.70 is 1 hour, and that's ignoring setup time, etc.
Meanwhile, on Lambda, you can actually run 1600 60s jobs (or 27 CPU-hours) in 3 minutes. This is inclusive of setup time, job submission, stragglers, etc. [1]
Of course, if you've got sustained load, it's cheaper to go with spot instances, but the "occasionally I need a buttload of compute," model is well-served by Lambda.