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Forum Moderators: rogerd
I'd suggest you look at it as an "overhead" expense and set your budget not expecting any immediate financial return. Set up tracking to see how many pageviews you get from these visitors and whether they register. If you track by keyword you may find that some terms that are more productive than others.
I'd guess very specific keywords like "Widget Blog", "Widget Forum", etc. might be good. I'd make the headings very obvious to avoid clicks from people expecting something else, e.g., "Widget Discussion Forum" or "Talk about Widgets" vs. just "Collectible Widgets". The latter might draw more clicks but also more visitors who back out after a single page.
If you aim at getting more users it might be a necessary or usefull investment that in itself doesn't bring revenue back directly (from ad-clicks/membership payments) but on the other hands adds to the value, quality and quantity of the site.
A large number of people who want to engage in a forum search the term "... forum". If you target the keywords well, you might get a fair number of people who are right for your forum.
One of the great things about adwords is that you can experiment with ideas for a relatively low cost. $10.00 will buy you 200 hits at a nickel a piece. As the other post said, be sure to track the experiment.