Why not just keep a PC running for this? Renting a EC2 is probably more expensive. This whole cloud computing is overrated.
I can't guarantee my own PC to be up and running at 99.9%+ all the time. But I agree EC2 is not the most efficient option for running an automated trading algorithm. Even if I'm only using it for less than 8 hours a day, 5 days a week, it is painful to restart the instance every day via EC2 if I were to hire it on a pay-as-you-go basis to save $$$. VPS is my preferred way atm.