It’s also worth trying to really do a research of what keyword phrases people actually use for your product/service and you need to find a niche among those in order to keep your cost low.
I don't want to spend more than $3 per a click and even that is a lot, since people search stuff by sometimes the weirdest terms, meaning cost per click can be a nickel. However, if you really want to target specific group of people you will need to spend some money and time in order to see what works for you. In addition, it’s an ongoing process anyways.....