Auto Sensors adjust automatically to the system to ensure that they do not incur more overhead than the percentage associated with the setting. It is not possible to tell how much detail is captured, because it depends on the complexity of the application or environment. You may need to adjust the resolution settings until you see the desired level of detail for a tolerable amount of overhead.
Check the following link about overhead in the Production and Non-Production environments:
In addition to what Babar said, you could also try taking a Thread Dump to see how much overhead the DT agent is adding. You'll see 3 threads related to the agent (these are dtautosensor, dtmetricstimer, dtcontroller) and you can check how much CPU time they are contributing. I know it doesn't exactly give you the percentage contribution you're looking for but it provides you a different option than taking a CPU Sample.
Hope this helps,