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In my city, the few BTMs have a fee ranging from 9% to 14%-plus. The fees were lower until recently. It may be that when you add up all the costs (the machine itself, managing it (putting BTC in, taking cash out, electricity, rent, etc.), a very low fee would not sustain such a business.
To my knowledge the machine I have used DOES NOT show the fee either.
20% is very high. Even the normal 14% I see drove me to using Circle when I want to buy.
The one time I checked out an ATM in my city, the price was 15% higher than the market price. I didn't bother with it, as there are much cheaper options.