it costs a couple of days discussing, a couple of hours thinking/reviewing, few minutes coding and a couple of days code review/testing. Period.
Regarding storage. Though I do suppose that you could instead change the draft feature to 100 unique thread replies/edits. (i.e. message ID or new replies to given threads)
OR we could just treat fresh posts differently letting them
1- To stay a lot more in draft status
2- being maintained as a single instance (overwriting previous instance)
While we keep a limited number of latest versions ( 3 for instance) of each (already saved) post edit history.
Such a policy would help keeping track of drafts for much longer periods of time and for more posts at the same time without new storage requirement. The key would be distinguishing between edit and new operations, not a big deal imo.