There's no fixed answer for this. I see some folks note that you need a big following - that's *absolutely* untrue for a niche market. Yes, very very small markets may not be worth the effort but you do not need a huge following to be successful. You can target your prospects very easily via Facebook.
Do your research - what's the demand for the keywords you are targeting? Why would people want to pay for your site? What value are you providing that isn't available elsewhere? Keep in mind that just a community likely won't be enticiing - you need content that isn't available anywhere else. Few people are interested in committing to forums these days.