I have checked three smaller machines.

On one (coreelec) systemd has 7M of resident data, of which 5.5M are shared libraries; by comparison, the numbers for sshd are respectively 6M and 3.5M.

On an OpenWRT machine without systemd (it's using procd) there are 700k of resident data. So the "cost" of systemd seems to be ~5M. Certainly I wouldn't run systemd on an old router with 16MB of RAM, but the cost is two orders of magnitude less than 1G-300M.