My vw rabbit gas is marked at zero yet I filled the gas tank

Sending unit is broke or rusted in place. These are in tank

Is it expensive to fix

In basic terms yes..part is $150 ish and labor is what gets you.

The fuel pump is underneath the backseat which is pretty cool disconnect the two lines you twist office snap ring type deal that holds it down look it up on YouTube you can change it yourself the hardest part is holding it down against the pressure cuz it's spring loaded to hold it against the bottom of the tank and then putting the ring on you got to make sure that you clean it off real good before you pull everything off the pumps are a little bit expensive sometimes you can find one for a little cheaper I found a Bosch for 125 it's normally a little bit more expensive whenever you put in original equipment OEM it will cost you a little bit more but it's worth it good luck remember that seals kind of loose fitting if you use the old one try and make sure you get it on your right it's cheap just to buy a new seal it didn't come with the new pump