Not pertaining to iOS specifically, but freelancing in general, also as a citizen/resident of the USA, I would charge roughly 4 times the hourly rate you get from an employer. I've seen other freelancers charge around this much.
So if an iOS dev is making 25EUR/hour, then as freelance, said dev would charge 100EUR/hour.
This may seem high, but there are few reasons to charge this:
- You have to bring in the same salary, so you will charge 25EUR/hour minimum.
- You have taxes, insurance, infrastructure investment, and other liabilities, this can come near your take home pay.
- You will not likely bill for full-time hours, so you have to account for all the unpaid work you will do. Freelancers do a lot, up to 50%, of unbillable work.
Of course there is latitude in how much you charge, but hiring a freelance shouldn't be assumed to be cheaper for the customer, or more/less lucrative for the developer. The benefit of freelancing is freedom in working how you would like work, or working with someone who can work with or around you more easily.