Not too sure about this, but 2 things I know for certain.
1 ) Each client uses about 8kb/sec .
2 ) Don't set your hopes too high. Even the most popular of servers such as Guildwars and Winters Roar hardly ever, if ever, get over 100 people on them.
You can afford $150 p/m ?! Nice.
I would say go for the least expensive one - people are able to run servers off their PC that they use for everything and support up to about 10 - 30 people.
Good luck!
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