That is a fairly standard price, however there are people out there that make them cheaper. We did Who's Afraid of Virginia Woolf a few years ago and were contacted by a guy who sold them for about ten bucks each. He apparently scours the internet looking for companies doing plays that he knows require breakaways. His didn't look quite as good as the $20 ones, but with a label on them and sitting with some real bottles, it's very believable.
Unfortunately.... I don't know who that was or how to contact him...
BUT I did just find this place online: http://www.newrulefx.com/SPbottles.html
that has 6 pack beer bottles for $60. plus shipping of course... but better than $20
There are also a lot of resources and info about making your own, so check youtube if you are up for that.