The mayor of a small town is probably the best analogy I've seen yet for running an online community. On top of that add in policeman, agony-aunt, therapist, decorator, architect, town-planner, accountant, friend and in my case fiance :-)
I don't know that you can actually "build a community" the best you can do is provide the infrastructure and support for the community.
When we started the site we'd never even heard of an online connunity, we were just playing with some chat software after being disappointed with our local ISP's implementation of it.
As for cost/benefit it's only since I can't afford to keep the place running on my own (the last year) that we've even started to think about that.
Anyway, a brief overview of our very organic (read disorganised) progress.
Day 1 - Started with chat which went down really well.
Month 6 - Users started "borrowing" other members nicknames so we added an optional membership system.
Month 9 - Continued adding members (including a few anti-social ones) so at the request of the users we added a few "chat guides" (watch for the politics when adding any sort of monitors).
Added member-member messaging so members could communicate without both parties being online at the same time.
Added forums which is quite popular, but someway behind the chatroom.
Started arranging night's out for the members around the country (Ireland).
Added "build your own website" features. Never really took-off so wasted the €800 that cost.
Added our own browser based email system, that didn't go down too well either (another €500 gone).
Added member profiles & photo albums, they seem to like that so we're looking at adding more features.
Added eSmacks & eKisses for members to send each other (I suppose you could call them emotiflashes).
Some of the members seem to get really into the "status" of being the most eSlapped/eKissed so we're going to be building on that in our next redesign.
Next few weeks will see us adding Video/Voice to the chat room as well as launching the Video/Chat instant messenger.