The year of your Python is 1978! The value, is whatever any willing buyer, is willing to pay, in any given geographical area! Here in the PRK(CA)I have seen Nickel Python's go from $1,050 up to $1,450! Other regional areas, might be lower in price!
I saw a 6-inch nickel as-new Python at a show today for $1100. I believe it is the same gun I saw a couple of weeks ago and it had not sold. In the last month or so, the asking price for as-new 6" nickel's seems to be in the $1100 to $1200 range. Not sure about one of lesser quality, but personally, I would want to deduct about 20% for a 98% gun and perhaps 30% for a 95%, but I know dealers don't want to hear that since they think they are made of gold.