I do free adverts and ask people to help 'share' the post but I have paid facebook to put my ad up as a sponsored ad and it worked amazingly well.
You can cap the amount you want to spend, tailor it to be seen by certain people ie: females, aged 25-55, living within 10 miles of you etc. It will tell you how many peoples newsfeed the ad will show in, (1000's) Facebook then put the advert out and within a few days my advert has reached it capped limit. I set it at £25 and did this 4 times in a month so spent £100.
In that month I had 47 new clients which is the most I have ever had in one month and turned over £846 based on those clients.
I offered a half price treatment and 29 of those new clients rebooked for their next appointment, I offered them 25% off there next one if they booked that day.
So although it cost me £100 for the month it was well worth it.
Try it once you can spend what you want cap it at £5 if you want and see what response you get.
Good Luck
Clair