1000 word messages = about 5000 bytes * 100 * 46854 members = about 22 gigs
50 word messages = about 265 bytes * 100 * 46854 members = about 1.1 gigs
elastic block store storage on amazon web services: about 10 cents per GB month
storage on relational database service on amazon web services: about 12.5 cents per GB month
for messages alone (the 'real' content of the forum takes up much more), presuming every single member has a full message box, with either one hundred 1000 word long messages, or one hundred 50 word long messages we're looking at: anywhere from $2.80 to to $0.11 per month on storage costs for messages alone! (not counting the price of the i/o usage which is 10 cents per 1 million I/O requests)
of course, the usage of the servers themselves costs a bit more.... (if we get performance issues for certain high usuage periods aws is the perfect tool to deal with it.. they even have products designed explicitly to handle that! 'seamlessly' of course, ..someone would need to code all of that up and do the actual work of making that a reality.... but this time of year people are busy doing other things... so, maybe, if anything, we could expect it after the annual staff retreat!
or we could just go into the phpbb settings and change it)