With both AppMon and Dynatrace the best you can get is a ballpark figure as far as estimating what overhead will be. For instance with AppMon with default sensors it was generally kept under 3% though the nature of the application and of course configurations could impact this.
The same would be true of Dynatrace, I haven't heard such an estimate for Dynatrace OneAgents (at least that I can recall) but the case would be the same that the only way to see how much overhead there would be is to compare it both before and after the agents are installed.
One key difference with AppMon is that in Dynatrace if you look at the hosts' processes you can actually see the resources that all aspects of the OneAgent monitoring are using. This should make it much easier to identify any overhead. Additionally note that since we can see the impact there are safeguards in place such as if network related monitoring overhead goes above a set threshold (I believe 5%) it will limit it:
"Additionally note that since we can see the impact there are safeguards in place such as if network related monitoring overhead goes above a set threshold (I believe 5%) it will limit it"
Is there any documentation supporting this? Some clients would like to see an official statement on this.
Yes, our official docs pages highlight this 5% cap. But it is strictly for the network monitoring agent. https://www.dynatrace.com/support/help/monitor/network/how-do-i-monitor-network-communication/
"Overhead generated by network monitoring is negligible and varies based on the analyzed traffic volume. Dynatrace monitors the overhead generated by network monitoring. If overhead increases above 5% of available CPU, Dynatrace, throttling occurs. The network module is then paused for slightly less than 3 minutes. After this time, the network is re-enabled. If the threshold is still exceeded, network throttling occurs again, with the network module paused for twice as long. This continues until the threshold is no longer exceeded. The timing of the pauses doubles each time that the network is re-enabled and the threshold remains exceeded, up to a maximum pause time of 45 minutes."