It may not seem like it, but ATM machines cost money. The surcharge is the only way for an ATM to exist or there would be no reason to provide an ATM. Thankfully, because of benefits and convenience an ATM provides, there is a certain price that provides good demand.
Since you do not want to scare people off from using the ATM, it is a good plan to figure out the minimum price you need to set it at to breakeven. Costs like maintenance, wages, and any parts should be factored in. Costs of owning an ATM varies, so figure out the breakeven price, then increase from there.
If placing an ATM machine in a place with good foot traffic, more people will use the machine. Customers that withdraw from an ATM are also shown to spend more than people who do not withdraw from an ATM, so try keeping track of average transaction value of your store. The more people spend at your store, the less you can charge while still making more money.
You also have to keep in mind the prices of other ATM machines in the area. It is easy to change the surcharge amount, so keep that in mind when basing prices of other ATMs. However, if your ATM is the only one in the are you could charge more than normal. But do not overcharge just because you can.
The main things to look at when setting your surcharge is how much you need to breakeven, then compare to other ATMs in the location. If you do these, you will see a lot of traffic that comes to your ATM.