It depends, 4chan ads aren't the most expensive thing in the world but you need at least $50-$70 for an ad campaign that will actually be seen by people. Moreover, you need to be advertising primarily to boards that will see appeal in your forum I.e. in your case you'd want to focus on mostly /r9k/, /v/, and /a/ potentially. You need to think on who exactly you'd want to attract to your site and focus your efforts on shilling to those boards.
Tl;Dr Not awful but you need to know your audience that you're shilling to.