It's between £20-£25 in my area. A few of the franchises have special offers, like 10 lessons for £99, and for one of them it's optional, one is compulsory (that I can work out) but another, the AA, appear to reimburse fully the DI for the discount. (Which seems reasonable.)
But yeah, with hourly rates like that, and franchise fees to pay, it's shockingly low earnings.
There are pros and cons, but I think the best way to maximise earnings is to go all-in and be independent and not get training from one of the big 6. It's about £2500 with them, whereas it can be found for nearly half that elsewhere.
But being part of a franchise for a year or so may be advantagous in gaining experience and creating a client-base - if you can survive financially, that is.