I'm not sure what list you were looking at, but I'm seeing many baby monitors for sale for under 20 dollars. And with 9 months to plan, I suspect that it is OK to get one shipped if it is not in stock locally. I'm just not sure how big the market is for people who cannot afford a monitor, yet live in homes so large that they cannot hear their baby.
I think deaf parents might be a better angle to take with this product, as it could give a non-auditory cue back to those parents.