There are three kinds of faded posters:
1) They faded in such a way that you can hardly tell, unless you see it up against an unfaded example.
2) They faded noticeably, but the new colors are still attractive, and people would still enjoy it.
3) They faded considerably or unevenly, and the result is an unattractive poster.
Since the ONLY way to restore such posters is to "dip them in paint", which is very drastic, I believe it should only be considered for type 3.
Since your poster is a type 2, why even consider it? I too like it the way it is, and you got it for a massive discount.
Incidentally, it WAS in very nice condition prior to the fading. It was on the wall of a studio exec's office for many years, and he likely did not know the danger of how he was displaying it.