What is interchange?
Interchange is the portion paid to the bank that issued your customer's card. It varies by card type, business category and how the card was accepted. A swiped consumer debit card carries far lower interchange than a keyed corporate rewards card. No processor can price below it, and the published tables run to hundreds of categories.
What are assessments?
Assessments are the card networks' own charges for running the rails. They are a small percentage of volume plus a fixed component per transaction. Like interchange, they are set centrally and applied identically regardless of who your processor is.
What exactly is the markup?
The markup is what your processor adds on top for providing the account, the gateway, support and risk management. On an interchange-plus pricing model it is stated openly as a percentage plus a per-transaction amount. On tiered or bundled pricing it is folded into qualified, mid-qualified and non-qualified buckets, which is why those statements are harder to read.
How do I find my markup?
Ask for it directly, in writing, as a percentage and a per-transaction figure. If your statement uses tiers rather than interchange-plus, request a re-quote on interchange-plus so the markup is visible.
A processor unwilling to state the markup separately is telling you something useful.