ps -o rss | awk '{mem += $1} END {print mem/1024 "M"}'
That much.
(Well, assuming you only run one Tmux session and are on OSX 10.5.8, otherwise: something like that).
I’m using a variation on that theme in my Tmux status-bar so that I know how much memory my session is using as well as the overall stats for the machine. This will be quite interesting for me; I’m window shopping and wondering what I could get away with (As I write this I’m using about 100MB).