I've checked this on my work computer, via the app and also via my home computer.
Booking a Basic fare from Dublin to Ibiza shows €82 one way and €52 for the return. See?
When you click continue, lo and behold, the price is not €134, it's €171 - which is the price for a Plus fare.
Not particularly great at all really. I called BA to report it and the helpful agent said she could waive the telephone booking fee but the lowest she could see was €163. She was definitely not interested in doing anything about it and suggested a ticket had somehow broken the pricing and I should wait until tomorrow.
Considering DUB-IBZ still shows 0 tier points in the calculator and now this, you have to wonder about things. I pity the poor person who actually gets stung by this creative mathematics.