Best way to value a forum?

I have been looking at buying a forum, and heard varying opinions on how to value one. To give some basic stats on the site:
A few years old, 500K Posts, 10K members, and approx $25K in revenue/year of which after marketing, advertising, SEO etc. they have $20K in profit. I feel I could do most of the SEO/Marketing myself which will save a pretty penny.
I have heard 1 year profit for a lot of websites, but forums are a lot less work(no content to generate), and more stable, so I thought…
Best way to value a forum?

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