You could use a service such as Ning that allows you to start a forum and social network for free and then you can pay for premium services when your site gets to that point.

Starting a forum is tough so there is no shame in testing a few ideas and learning the ropes on a free service. Once you understand what you are doing you will probably have a better idea for a new forum and that could be the one you invest more money into up front.

I agree - free or freemium platforms allow you to test ideas/tactics out and when your community grows, you can migrate the community.
A word of advice on growing any online community is that your need to obtain a critical mass/membership and keep them coming/engaged.