The concept of "Annual Percentage Rate (APR)" is expressed in American Sign Language (ASL) by using the following signs:
ANNUAL
Usually you can just sign: "ANNUAL PERCENTAGE" and then spell R-A-T-E. If the person isn't already familiar with what an "APR" is, then you will need to elaborate on what you mean by "rate."
The concept of "rate" (as in "annual percentage rate") doesn't have a "specific" sign in ASL. The word rate has several meanings and each of those meanings is signed in different ways. In general for the concept of "annual percentage rate" we would express rate as a combination of the signs "YOU PAY HOW-MUCH" and then add a version of the sign "MORE-OR-LESS"-[amount]. Thus the sign for "rate" (in this) case is a combination of the context of "making regular payments," the sign HOW-MUCH, and the sign "MORE-OR-LESS."