Another FYI, the companies that do this type of service are called Merchant Services, and they all have fairly complex billing and pricing plans, which in my opinion are intentionally designed to make it virtually impossible for you to compare apples-to-apples when comparing companies.
Most of them provide some type of monthly base charge along with their transaction charges, but they also use thresholds for total dollars charged to alter their transaction percentages, and there are different transaction percentages for each of the various input forms (Swipe, Swipe with Sig, Keyed, Internet, etc) and different thresholds for each of the input forms.
But in general, a good rule of thumb is that it's going to cost you around 2% on each transaction. For companies doing very high numbers of transactions or dollar totals, there are complex programs which can be used to analyze patterns and select plans, or to dynamically modify plans based on monthly activity. Unfortunately, my business is not one of the "big boys", so we have to suck it up and take what we can get. But I bet Amazon gets a much better rate on transactions and their Merchant Service providers make it up on volume.