$250 would be a good approximation, any additional add ons and licensing would probably cost a lot more.
The project im currently developing has cost me 400+ and thats without a custom layout as i will be doing that myself. So id estimate around 200 for a basic forum.
GameRenders cost absolutely nothing to setup when it was first launched. I originally used free hosting, and I found someone to buy the domain gamerenders.com for me. I originally used phpBB which was also free, and spent no money on advertising. Infact, since I did not have a credit card or Paypal account at the time I couldn't spend money on anything at all. At the same time, someone could easily spend a lot of money buying an IPB licence, hosting, a domain name, a website/forum skin designer or coder, money on advertising etc but not make a successful website that people visit or register on. So the moral of the story is that you don't need loads of money to make a successful forum, just a good, original idea
agrees with llama start off with hosting - $0 find a mate or something forum - $0 not all forums have to be ipb design - $0 diy again with the coding domain - $0 if u go .tk or buy one using cred or $2.95 for a .info total cost: $0 - then get some advertising, save up viola Daz