Ford Focus zts 2000 gas gauge not working?

[magy]

car ran out of gas...i went and put 2 gallons of gas in the car and it still wouldnt start the mechanic said it was the fuel pump so he replaced that----charged 250 and now when we went to pick it up the gas gauge was not working. we asked him what had happend and he said that it wasnt his fault that that was already there when he got the car......i know it was working because when i put the gas in the car it marked the right amount , now we went to fill it half-way and it still marks that it is empty and the gas light is on.....mechanic refuses to fix it says he even had an electrician go look at it and the electrician says the problem was likely already there and that the circuit board is burned [whatever that means] do any of you know how much it cost to replace that thing.........