Standard cost-benefit analysis does not apply to stress or load testing. These tests typically cannot be reproduced manually, so no comparison can be made. These costs typically represent the impact to the bottom line with these automation tests. There are some benefits of automation that are very hard to put a price tag on. Never the less they should be considered.
Automation removes the mundane aspects of testing.
If the less dynamic areas of an application are automation tested, then by definition, the more dynamic areas remain to be tested. More complicated exploratory testing can be applied to the dynamic areas. The expected result is that a smaller, yet more technically adept testing staff will be required in this environment. This should result in a more loyal and interested staff. It is hard to put a price on this, but the benefit is very real.