When you have 400 members. you must say that 20 of them will be always online.
If the community is highly activ.
So in any way - online the performance counts when lot of people are online - because offline users don´t need much resources (maybe sql queries) - so if you 300 users Online and it costs 400 a month - that will mean, that you CAN`T run your community without letting your members pay or have an other good finance strategy.
So if you are expecting to run something like facebook - where many users are online at the same time and for the most hours of a day, - maybe UNA is not the best solution.
That´s what someone could read out of this post.