My own research into them was they charged £6 for an unemployed/student member, I would imagine they would look for a similar percentage of the salary as BALPA for those working.
In fact the full membership rate for employed members is as follows:
Category 1 (Full IPA/IPF) £14 monthly.
Welcome to the IPA
As far as BALPA being the only choice available is concerned - It was Mandy Rice-Davis who once said 'They would say that wouldn't they?'
Just have a look, no outlandish promises, posted in the interests of a balanced view.