What is the value of a 30-30 gun?

It's not possible to answer the question as asked. There are too many types, models and variations of these guns to consider. These guns can sell for as little as a hundred dollars up to thousands, depending on what it is, when it was made and condition. Condition is everything when appraising a gun. As always, it's recommended that you have it appraised at a Gun Shop or qualified professional.