The most important factor is what it cost to get yourself a license as money transmitter (and there may be other licenses you might need to obtain), which you have to renew annually. It all depends on the country and state you operate in, but it could go easily over $100,000 a year.
In that regard, it shouldn't come as a surprise that a 4-5% margin is somewhat of a minimum in order to make it worthwhile, especially if you deduct taxes afterwards.
In the Netherlands where it is quite competitive, net profit looks like 1.5-2% on gross transaction volume.
That's pretty harsh, but hey, entities deploying ATM's know what they get themselves into. That's why most of the larger entities try to deploy as many as they can, because that's the only way you can have it work for you instead of against you. The cost of these licenses is a killer, literally. It's meant to demotivate smaller businesses from entering a space the larger businesses don't want them to operate in.