This article has some info about Raspberry Pi power consumption. It appears that the Pi3 draws ~250mA when idle. I would just leave it on.
The little dingus you linked to will do the job, but will eventually lead to card corruption when the Pi is turned off while writing to the card. You can diminish the risk by using a distro like DietPi, which logs to RAM and does not write to the card at all when used as a RoonBridge.