Those folds are very valuable errors. Prices are all over the map. Plus your fold is very large which makes it worth even more. I can't give a price $200? $300? more? Error notes can be difficult to price.
The way you stated the your sentence makes me wonder.. Do you actually have this $20.00 bill it you just saw it? and.. That you just want to know how much an Error like this would be worth?
Error notes like this do command a premium. It's a folding error. Since it's from 1977, how long have you had it or do you even own it? It is in very good condition.