Answer:
There are a lot of factors to determine the value of autographed memorabilia, let alone autographs of the deceased. Most importantly, is the autograph certified-- proven authentic, either by certificate of a legitimate collector or a picture of someone getting a piece signed. This will separate your autograph as a true and valid collector's item. Secondly, the condition, size, and year of the car can and will increase the value.
A Dale Sr. certified autograph alone on an 8X10 can be $700 ballpark. Add a collectible race car with it that and you could be into $1000 if it's coming from a legit source.
Final word: As a seller, do your best to prove the authenticity. As a buyer, beware of those sad souls trying to make extra money off of this legend.