3C Toolbox Benchmark on HTC Sensation XE
This benchmark has battery monitoring is on at all times. Recorder set to record everything at an interval of 20 minutes. Extracted 4 or 12 hours when phone was mostly in plane mode, so that the app UI is not used, eg measuring monitoring/recording consumption only. CPU is set to hotplug during standby, eg single core running.
It shows that at an interval of 20 minutes, 3C Toolbox consumes about 27 seconds of CPU per day to record full battery history, application data (network, memory and CPU) and all component states.