You could easily start up a forum without paying a penny, the thing is, it wouldn't do very well. If you were working on a minimal, but available, budget, I would say...

$3 a month for hosting = $36 a year
1 year VB license = $85
Ads/Contest (you need to have ads and you should have a contest): $50 (that's the minimum for what I suggest)

For a grand total of about $171 for the first year. Until you start making a bit of extra money, I suggest you stay away from buying a custom VB skin. Some free skins can be good enough.