Hi there @Arthur! Welcome to the Airtable community :slightly_smiling_face:
Try this…
TRIM(SUBSTITUTE(SUBSTITUTE({Prix}, "€", ""), ",", "."))
The TRIM() function will remove any white space (looks like there is a space between your currency symbol and number). You could also use…
VALUE(TRIM(SUBSTITUTE(SUBSTITUTE({Prix}, "€", ""), ",", ".")))
To convert the value to an actual number (as opposed to a string) if you’ll need to perform calculations with it later on.
Hope this is helpful!