I wouldn't really think about it. If I was that desperate, it would be because my forum wasn't good.
i never pay for my websites until they are active and popular
I can't see this forum investing to SEO.
Most of the vBulletin converts have given us their tips and all that. I find the little things they do are better off than MinervaSEO and CommunitySEO. Also, IPB 3.1 will have a lot of built-in SEO features, so I'm really excited for that.
phpBB and MyBB both have fantastic SEO options as well. Both are free too.
Haven't spent a single cent, but a domain would probably be part of SEO, as its a SEO domain.
I dont spend much, it depends on how much i earn from the website. Normally i would invest 20% of the revenue from this website in SEO