============ORIGINAL============
FOR /f "tokens=3 delims= " %%a in ('"dir %drive%\|find "bytes free""') do set free=%%a
SET free=%free:,=%&         rem eliminate commas in number
SET free=%free:.=%&         rem eliminate dots in number
SET free=%free:~0,-3%&    rem divide by 1000 by cutting off the last 3 digits
SET min=13300000&                      rem minimum 40000000 bytes
IF %free% LSS %min% (
echo Not enough space available. You must have 13.5gb of free space.
echo You have %free% when you need %min%

=============EDITED=============
:: FOR /f "tokens=3 delims= " %%a in ('"dir %drive%\|find "bytes free""') do set free=%%a
:: SET free=%free:,=%&         rem eliminate commas in number
:: SET free=%free:.=%&         rem eliminate dots in number
:: SET free=%free:~0,-3%&    rem divide by 1000 by cutting off the last 3 digits
:: SET min=13300000&                      rem minimum 40000000 bytes
:: IF %free% LSS %min% (
:: echo Not enough space available. You must have 13.5gb of free space.
:: echo You have %free% when you need %min%