Since it scales somewhat close to linear, you can just calculate it yourself.
And since you pay per minute on EC2, you can either chose to have 10 instances calculate a minute or 1 instance for 10 minutes. Either way, you pay for 10 minutes of processing time.
p.s. simplified but probably somewhere in a reasonable ballpark
It's actually rounded up to complete hours, so 10 instances for a minute is 10x the price of 1 instance for 10 minutes.
For fast iterative development this isn't an issue - you can re-use the job flow (the instances you spun up), so you can launch the job several times over the course of that hour.
And since you pay per minute on EC2, you can either chose to have 10 instances calculate a minute or 1 instance for 10 minutes. Either way, you pay for 10 minutes of processing time.
p.s. simplified but probably somewhere in a reasonable ballpark