As others have said, the purpose of insurance is to put back to the level you were at before the incident - NOT to put you in a better position after it.

Most states say that insurance companies have x amount of days to respond. The norm is between 21 and 30 days.

If you think they are being unfair, you can file a complaint with your state's deparment of insurance, provided you didn't sign anything accepting the amount they offered.

You can also contest the offer by showing proof (ie - printouts of online findings or ads from the car section) how much it will cost you to by a car that is the same make, model, year, and has comparable mileage. It's not a guarantee but we have done this as well when one of our cars was totalled.