Tom
First, welcome to the forum.
I don't know if the style of the 65 GS can be improved on, other than a set of American 5 spoke wheels.
As far as a diff. in $$ between the GS & Skylarks And a Chevelle, GTO, I guess it was in the original promotion from the factories and dealers. The Buick was seen as an "old mans" car. The other GM divisions pushed performance. Also a lot of people have more $ than common sense.
I traded a 66 GTO hardtop for my 65 GS conv. in 1969 and have never regretted it. That Goat was a piece of @$((&^%$#.
Al