There used to be different fines for not answering and for incorrectly answering, but my 30 second google search found an updated single number of $5000. Not sure if that is a single fine or if it adds up for multiple offenses with the same form.
Last time around, in 2010, I remember a lot of angst by some people who received the long form of the census, which is very invasive/nosey.
Not sure if they enforced against anyone, though. A lot of outlets say the law is unenforced unless the government wants to target people for some reason....prosecutorial discretion, you know.