If you are a congressperson or a senator and you have an idea for a new piece of legislation, at some point someone will have to tell you how much it costs. But, how do you put a price on something that doesn't exist yet?

Since 1974, that has been the job of the Congressional Budget Office, or the CBO. The agency plays a critical role in the legislative process: bills can live and die by the cost estimates the CBO produces.

The economists and budget experts at the CBO, though, are far more than just a bunch of number crunchers. Sometimes, when the job is really at its most fun, they are basically tasked with predicting the future. The CBO has to estimate the cost of unreleased products and imagine markets that don't yet exist — and someone always hates the number they come up with.

On today's episode, we go inside the CBO to tell the twisting tale behind the pricing of a single piece of massive legislation — when the U.S. decided to finally cover prescription drug insurance for seniors. At the time, some of the drugs the CBO was trying to price didn't even exist yet. But the CBO still had to tell Congress how much the bill would cost — even though the agency knew better than anyone that its math would almost definitely be wrong.

Today's show was produced by Willa Rubin and Dave Blanchard, with engineering help from Josh Newell. It was edited by Keith Romer and fact-checked by Sierra Juarez. Jess Jiang is our acting executive producer.

We want to hear your thoughts on the show! We have a short, anonymous survey we'd love for you to fill out: n.pr/pmsurvey

Help support Planet Money and get bonus episodes by subscribing to Planet Money+ in Apple Podcasts or at plus.npr.org/planetmoney.

Always free at these links: Apple Podcasts, Spotify, Google Podcasts, NPR One or anywhere you get podcasts.

Find more Planet Money: Twitter / Facebook / Instagram / TikTok / Our weekly Newsletter.

Music: "Back in the Day," "What Da Funk" and "Parade Floats."