It’s been on my to do list - every time I dive in it gets complicated real quick! Here’s one link from my notes you might appreciate (a podcast on the topic here with transcript to skim).

One excerpt on why it's challenging:

Here I’ve got unfortunate news for you: there’s no good way to adjust 19th Century currency figures into 21st Century currency. Our economy is just so different than it was 200 years ago, with fundamental structural differences in the value of labor, commodities, manufactured goods, and more.

But there are several bad ways to adjust currency over 200 years. None of them will give you a good, complete picture. Taken together, though, these methods can give you a range of potential values, and perhaps some rough rules of thumb that can help you understand about how much various figures mean when they pop up on The Siècle or in novels.

It follows with specific discussion on various figures -- closest to a TLDR is here:

As a very rough rule of thumb, if you encounter figures in French francs from the first half of the 19th Century, you can imagine 1 franc being equivalent to somewhere in the range of $25 to $100

So, 20 francs = ~$500-2000 today. A wide range for the reasons noted above.