No way would I pay £20 per DC for a 1hr group class and I am used to exorbitant inner London prices. Not even your OU tutor would have been earning that much to teach you .
Totally agree that 11+ tutoring is different - the cost of tutoring are offset by the savings made if the DC gets into grammar school rather than private. Having said that, I'd balk at £60, that's rather more than anyone I know has paid. Creative writing is not part of the test. It's not an essential life skill. It's a hobby so should be compared to the cost of ballet, music, drama, gymnastics, etc.
E.g. recently there was a creative writing workshop held near here for kids 10+, 2x 2hr sessions held on consecutive days with well-known authors. £40 for the course, so £10 per hour. I'd say that's on the upper limit but totally acceptable as a one-off holiday thing.
The issue is not what it's worth to you, but what it's worth to the parents - not even the DC at that age. If I was to choose between 10 ballet lessons or 3 creative writing classes, then there's no contest at all. I probably wouldn't pay for creative writing classes anyway though, despite my DD's enthusiasm for creative writing which I encourage and my own career in teaching literature & writing. I think that you would have to factor in that kind of attitude, so perhaps lower demand than you might think amongst your target audience because many of us who value creative writing value the creativity more than the technical accomplishment in children (based on lengthy discussions with friends in similar careers).
You might be better off looking at a slightly older age range and doing one- or two-day workshops instead - overheads would reduce significantly if you were hiring for a single day (or even half-day) rather than hourly, especially if you take into account travel time etc (yours and the parents who have to ferry the kids... Teenagers could make their own way there). I'd be more willing to stump up cash for a teenager showing interest and aptitude than for a 10yo for whom I am also paying for after-school activities every day of the week, or childcare.