a valid question.
APPLE SSD TS128E Media = formatted capacity 121.33 gb
go to Applications/Utilities/Disk Utility so see the info for yours
or 'About This Mac'/Storage
my Applications, Library and System Folders take up a total space of approx 12gb - with only what I use installed - the rest deleted.
so my free space would be 99gb before adding Documents, Music, etc.
at 97gb you're doing ok with all the standard Apps installed
to reinstall Lion and then the Applications you use would not save you a whole heap of space.
a slim external drive is the go