On a product that costs $5? Forget it. No-way you will get a return. (Background: making a living selling software online for 16 years, advertised continuously on Adwords for nearly all of that).
If you optimize so that you are paying maybe 2-5 cents a click it really just depends on the application and industry. Something people need to buy right then and there is going to have much better conversion than something they need to research and think about it. Plus there is value in just getting the name of your software out there and into peoples hands so that it then spreads organically after that.
For something like this I would probably start at like 30$ a month and keep optimizing - worst case you lose only a few hundred over the course of a year.
