Another of the inexpensive but highly effective marketing ideas published years ago by Bruce Milne. Maybe this could this work for you...
The trick here is to be very highly targeted in terms of audience, publication and, often, season. For example, if you produce tax preparation software that has modules for various states, a 1/8 or 1/4 page ad in the state CPA society publication in November (before the tax season) could reap large benefits. Most companies are aware of the big, national publications, but you may need to do some research to uncover the newsletters and local publications that have much lower rates but percentage-wise may target more of your found market.