Keep in mind that interchange fees are your bottom line. While Visa and MasterCard have small ticket interchange, these rates only apply card-present transactions.
Here are the Visa interchange fees for the US and Europe, for example.
Intra Visa Europe interchange fees
Visa U.S.A. Interchange Reimbursement Fees
The rates/fees listed on these interchange fees are your starting point. Add to them about $0.02 for assessments and your base cost is roughly $0.12 per transaction here in the US.
A merchant service provider's markup will be added to that $0.12. A competitive transaction fee for a card-not-present merchant on an interchange pass through pricing model would be $0.05 -- again, very competitive.
That makes your best case scenario about $0.17 per transaction said and done.