Inform the Public
If you meet in a public place, as opposed to a home, your club has an opportunity to publicize itself at every meeting. A local pub or diner may allow you to place a poster in their window (even if only during the meetings,) or on a bulletin board.
Some libraries may let you meet for free, but might restrict food items. In any case a public library might have a bulletin board for meting notices, or even a showcase where you could put a display of Esperanto items. Be sure to include contact information in the display.
In some parts of the U.S., (or at least in my part or New Jersey,) there are free newspapers which love to have stuff to print. A notice sent (preferably with a photo of your group,) might just make it into the paper in a few weeks. These free papers are very local (about 5 in my county alone,) but are often owned by the same company. If you happen to be involved in the public in some way, you might even have a reporter contact. My guess is that Esperantists might be considered a human interest story.
Some very local TV stations may want people to interview. It's definitely worth looking into as often these features are repeated over several days. Know your subject and have positive things to say about why you enjoy speaking and using the language.
If you hold any kind of event, such as a Zamenhof Day get together, definitely submit the news (with a photo if possible) to the E-USA Bulletin (E USA.) That way, the news will get out to any E-USA members in your state that things are definitely happening around them. Perhaps it will spur them to join you next time.
Finally, unless your group has an income, paying for an ad on a regular basis is pretty much a last resort.