As @dankcushions said, it's a percentage, so it depends on the total size of the file system.

This line of commands prints the reserved space in MB for the root file system:

sudo tune2fs -l $(df | grep -E '/$' | cut -d\ -f 1) | egrep "Reserved block count|Block size" | paste -sd\ | awk '{print ($4 * $7 / ( 1024 * 1024 ) ), "MB"}'

source: https://superuser.com/questions/444269/display-filesystems-free-space-available-to-the-root-user

edit: I just found out that it has to be modified to work on RetroPie.

sudo tune2fs -l /dev/mmcblk0p2 | egrep "Reserved block count|Block size" | paste -sd\ | awk '{print ($4 * $7 / ( 1024 * 1024 ) ), "MB"}'

/dev/mmcblk0p2 should be the root device of RetroPie on a Raspberry Pi. You can change it to any block device with an ext FS to see its reserved space.