Just for curiosity. An example machine: an dedicated amd64 server with the last stable version of FreeBSD and UFS for the partitions.
How much resources consume FreeBSD for each empty jail? I mean, I don't want know what is the resource consumption of a jailed server or whatever, just the overhead of each jail.
I'm especially interested on CPU, memory and IO. For a few jails the overhead is negligible but imagine a server with 100 jails.
Update: I've found a amazing blog post related to this question http://ivoras.sharanet.org/blog/tree/2009-10-20.the-night-of-1000-jails.html
The command issued will run along side the command chroot and thus could be profiled. It is probably different in each case, as each chroot would have different requirements and software inside said jail. Each chrooted user is doing different things and thus it should really be different per server daemon running under a chrooted user.
A simple test would be to chroot a user into a basic chroot jail and use another user to determine the amount of system resources used, using basic tools such as ps and top.
This isnt quite a question that can just be answered. its best to attempt to profile it yourself if you are curious. My guesses would be very negligible in comparison to the actual running daemons.
Good luck. Hope it helped