Using Lambda for the slaves is probably not the best idea because, as @saynay mentioned, you couldn't run builds any longer than a few minutes.
You could "wrap" a Jenkins fleet running on EC2 with Lambda functions to set up on a build request event and tear down on a build finish event, and theoretically this would reduce the amount of resources you consume if your builds are few and far between. Think of GitHub webhooks triggering Travis CI as an analogy.
Whether or not that would actually be useful is debatable.
You could "wrap" a Jenkins fleet running on EC2 with Lambda functions to set up on a build request event and tear down on a build finish event, and theoretically this would reduce the amount of resources you consume if your builds are few and far between. Think of GitHub webhooks triggering Travis CI as an analogy.
Whether or not that would actually be useful is debatable.