It's a great way to advertise for FREE, yes you don't make much money from them, but those kind of companies are just to build your business profile, not your bank balance.
You have to offer half price and they will take 40% of the remaining half. Don't be put off by that though, look at it as a great marketing tool, your offer will be in the face of everyone in the area who use that site, and most probably who didn't even know you existed.
You can do an "up sell" once they are in the door, if they have half head foils, upgrade to full head, if just a wash, cut and finish, up grade to a conditioning treatment - treat them well and some WILL return,
I've done one, they are great for quieter times like October and January.
Also look at " The Spree Book" you may have an alternate version in your area, they take nothing from you, you put your offer in, and people buy the book which will contain lots of money off vouchers..