If you're looking to start a new forum, chances are that you'll probably have to have a little bit of money. Whether you have a lot of money or just a little money, you'll have to pick and chose where you want to spend it. Ads, paid posting, paying staff, software purchases, and other various things can add up quickly. If you don't have a plan, you have a good shot of spending all your money too soon. With all that being said: How do you make your budget? Where do you put your money in the beginning?
I think this depends on the project. If it is a project that I think has more potential, then I will have a bigger budget but if the project has higher risk, then I will have a lower budget so this is more of a case to case basis. Furthermore, I would put the money on the forum software, domain and hosting. I will then use the remaining cash to buy the most important addons, start an advertising campaign and SEO work. Again, how much goes to which part varies from project to project.
Fortunately, I've been able to raise two successful forums without having to shell out. The only things that I need to keep paying for are hosting and domain names. I was never a fan of vBulletin or XenForo.