A dedicated server of ~450$ can easily handle 600-700 users. Though, i know people with 1000-1500 users playing on their server paying monthly ~10000$ totally. (dedicated server, advertisement, developer and for the rest, extra tools).
A "useless" advertisement costs 50-75$. A normal advertisement costs about 150$/m, a high "level" advertisement may cost even 2000$.
IMO you can handle 400+ users even with a ~150$ dedicated server, but you cannot find any that cheap DS with DDOS protection hardware, that's why most owners lookin for "expensive DS's".
I know, my reply is off-topic but w/e, i saw u talkin about dedicated servers and such.