From the Amazon page its $0.015/hr:

With Amazon CloudWatch... at a rate of $0.015 per hour for each Amazon EC2 instance you choose to monitor...

As an example, a developer may want to monitor 10 Amazon EC2 instances 24×7 for a 30-day period. The Amazon CloudWatch cost would be $108 (or $0.015 per Amazon EC2 instance hour x 10 Amazon EC2 instances x 24 hours per day x 30 days)...

That is close to what it costs to run a single instance to collect all your statistics (75$ if its small + your time is worth something!).

Yeah, my bad. Still, I think it's quite expensive. If all they are offering is some simple runtime stats, then a 15% surcharge on the cost of a small instance is pretty expensive. If you have 50 instances, you're paying $540/month just for monitoring.

That is close to what it costs to run a single instance to collect all your statistics

Assuming you need a completely separate instance which couldn't do anything else but monitoring, which is unlikely to be the case.

Think of it as a bailout for the third-party cloud monitoring startups; at least they have a chance to be cheaper than Amazon.

It is 1.5 cents per instance per hour. ($0.015)

[source : http://aws.amazon.com/cloudwatch/]