I had to use a different number in awk to get the free memory value, but here is a workable solution for you:
Script argument line
bash ${SCRIPT}
Body of Script
#!/bin/bash
memory=`free -m | grep Mem: | awk '{print $4}'`
echo "\nStatistic: $memory";
echo "\nMessage: Free Memory is $memory Kb"
fi
On your results - you can choose the values to warn if less than whatever number you deem suitable.