I never ever bill hourly. 90% of the work I do is value-based pricing, with a weekly or monthly rate once in a blue moon. My effective hourly rates are much, much higher than anyone would pay me strict hourly.

Also, I'm not sure where you're finding clients, but the market for senior iOS dev definitely is nowhere near $40 / hr. If you're competing with offshore devs on Upwork or people who do "web, iOS, Android, and anything else development!!" then yeah, maybe. But there are tons of clients out there who need senior talent with deep iOS expertise and professionalism and pay north of $100 / hr, or much more if you price based on value.