pgrep apache2 | xargs -n1 -I{} cat /proc/{}/statm | awk '
  {size+=$1; resident+=$2; share+=$3; process_count+=1} END {
    printf "%.2fn",(resident - (resident/size) * share)/process_count}'

/proc/[pid]/statm
    Provides information about memory usage, measured in pages.
    The columns are:

                  size       (1) total program size
                             (same as VmSize in /proc/[pid]/status)
                  resident   (2) resident set size
                             (same as VmRSS in /proc/[pid]/status)
                  share      (3) shared pages (i.e., backed by a file)
                  text       (4) text (code)
                  lib        (5) library (unused in Linux 2.6)
                  data       (6) data + stack
                  dt         (7) dirty pages (unused in Linux 2.6)