I need a calculated field that will capture a different billing rate per year so we don't lose historical billing data

2 years ago we had $X rate for billing per role. Last year it went up by $x. This year it will go down by $x. I don't want to lose the historical billing rate information, so I'm trying to figure out to write an if statement on a custom form - something like if (year=2019, billing rate = $x), if (year = 2020, billing rate = $x, if(year = 2021, billing rate = $x), etc. Of course that isn't the right syntax - I'm just trying to figure out the logic. How would I capture this in an if statement for a calculated field in a custom form? I'm stumped. I need your expertise please! 😊