I try and boost at least half of my posts every week, so that's three or four.
I pay £1 per boost, I don't think I need to do any more than that. Doing that, along with sharing my posts on local selling pages, I get about 700 post views. I get a few enquiries and have had a few bookings as a result of the boosts so they're paying for themselves so far.
Like Banner Penguin says, there's no other way you can advertise so cheaply and have such a big readership.
Sometimes it's not about the amount of immediate business you get from it, it's about getting your name out there and not letting people forget it. People may not need you there and then but will remember that post they saw for when they do need you