I don't think you will ever get it right but maybe you should look at all types of members and how they sell birds.
We all know or if not soon get to know the genuine fanciers who are willing to help other members or the site, the ones that only appear when they are selling or wanting something, the ones that sell the odd spare bird or the feather merchant.
I think £25 for 12 adverts is really cheap and my I suggest it should be more, anyone selling that amount of times is not just selling his excess stock he is verging on being a dealer but not always the case.
A tier or some would say a tear system would be better to cater for all or most and not just some would be far fairer and would cause less grief and moaning :-/
JMO