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It costs a lot of money to manage and run a forum. Apart from domain and hosting, you also need to pay for software subscriptions, addons and themes. How important do you think is it to generate revenue on your forum so that you can maintain the running cost?
If you are not making money from your site, I do not think you will be able to build it into a thriving community as you will need to invest heavily to build your forum into a thriving community