I'm just starting up my own gardening service (in Cheshire). I have invested in my training (spent last year doing practical training at a stately home garden) and I am qualified with RHS Level 2 (for which I paid to do the study at college for one year). I have to spend on insurance ('what if my rake went through your window' scenario), decent tools, vehicle and trailer, NI, work gear (incl safety gear), professional subscriptions, ongoing training, books, etc.
I suppose it depends on what you want from your 'gardener'. Hoe, mow and hack or understanding of your plants, how they grow, how to plant, prune properly and recognising the weeds! etc.
If a client wanted me to take away green waste that they couldn't compost or put in their council bin, I would have to charge extra as the council tip charges to dump green waste. Also it's in my gardening philosophy to compost so I'd urge clients to do same.
Anyway, I'm thinking of charging about £14/15/hour (especially after reading this post!
eeer, anyone looking for a gardener near Chester?!