what would the cheapist performance up-grade for a 1990 banshee be?

 

The cheapest bang for the buck is Pipes. It makes the Banhsee a whole 'nother animal with pipes...

RoostKing...

 

I totally agree with the pipes suggestion and I would do that first, but I think that the cheapest hop-up would be a set of Boyesen Power Reeds on your stock reed cages. It definitely won't give you the performance boost that the pipes will but you would notice a mild acceleration change in low and mid range.