Optimizing agent startup (costs) in Amazon cloud?

We set up Agent Cloud with EC2. That works. But something was a bit surprising.

Situation:
TeamCity has 5 build agents, all running in the cloud. No agent is currently running, means they are shut down.

Now 5 builds are triggered, building them takes 30 seconds for each.

What does TeamCity do? It starts all 5 Amazon build agents in the cloud. The first hour has to be paid allways on Amazon.

So we pay needlessly 4 hours to Amazon since TeamCity started all 5 build agents.

Is there a way to optimize the costs?

For instance there could be a delay time for the build agents: Only if, lets say for 10 minutes there is no enough agents, another agent is started. That could be a hard coded wait time or TeamCity could calculate it if it knows the average build times.