I am not sure if I got the numbers right but, assuming constant load of 20K users (not realistic but what the heck) pinging the server every 5 seconds (did anyone send me roses? no? did anyone send me flowers? no? so on) with each ping weighing about 1K, around 10-20 terabytes of traffic per month would result which would cost around $3-5K/month.
Other option is to get 10 dedicated servers, each with 2T bandwidth/month budget and handling 2K concurrent users, would handle the traffic and load handily. At $200 per server per month, price tag is $2K/month, cheaper than EC2 but more manual labor is needed to scale up.
Considering that 20K concurrent users is in the ballpark of Second Life, I think these numbers are good but they are specific to my apps which is task-oriented. For presence-oriented apps like Pownce, numbers would be much worse because they would have massive number of simultaneous users, even at lower ping rate (60 seconds for Pownce I think).
BTW, I am notoriously bad with numbers. This post is a disguised attempt to uncross my eyes with readers' help. ;-p