When Square published its card processing margins in quarterly reports, they kept about 1% - roughly a 33% margin. They have since stopped publishing this metric.
For instance here, page 7 [1] in Q4 2016, Square reported a 1.04% transaction margin against 2.94% transaction revenue.
[1] https://s29.q4cdn.com/628966176/files/doc_financials/2016/SQ...