This was the same at my school; you pay the dual rate while training - regardless of dual or solo.
Bearing in mind that, dual or solo, you get 15-30mins of brief/de-brief either side of the paid flying time I don't think that's a rip-off. Added to that, for solo circuits he certainly didn't go off to do paperwork because he never missed a dodgy landing
The extra charge for training on the hourly rate for a C152 at my school was about £26 plus vat. Considering that for every flying hour paid for there is about another 45mins to an hour of ground briefing this is a darn sight cheaper than an equivalent amount of driving tuition, guitar tuition or language tuition.
To my mind, that doesn't sound like a bad deal for the student.