I was a member a few years ago, and it was a flat fee then - the only discount I got was "no joining fee", which they said saved €250! I got a leaflet in my door regularly offering this discount (if you joined within 2 weeks that is!).
If you do join, be very specific about what they can use your mobile number for - I received text messages every 6-8 weeks for the past 3 years asking me to return (I moved away from the area so was definitely not going back and told them at the time I left). Eventually managed to speak to someone last time a text arrived and they said they'd take me off the list.