The going rate for a proffesional gardener?

Hello there, what is the going rate in your respective neck(s) of the woods to pay for a qualified gardener/horticulturalist?
Round here (edinburgh/scotland) it seems for someone to do simple tasks it's about 10 per hour(us$17) and 20 for more specialized work. With larger firms charging more like 30.
I am working as a jobbing hourly rate gardener for 15, and hope to get my RHS Level 2 hort' exam in feb after which I shall put my prices up to 17.50....
Does that seem too steep? It's hard to run a business on much less. Regards Benji

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