Perhaps something is worth to you what you pay for it? Though the commercial value of these girls will be next to nothing, a tenner for three birds is hardly the end of the world. After all, if their new owners won't even value them that much, the cat food factory might be preferable IMHO! So, as long as the charity is properly run (i.e. nobody's on the make), where's the problem?
Good luck with the rescue edandfred - I hope they all find good homes!