There is at least one incredibly rare guitar in that scene. The Surf Green Bass IV is proper vintage, and is one of only two ever made in that color. Chances are the rest of the guitars are the real deal as well.
As I recall, either earlier (or later) in the scene, Reiner asks him about the t shirt and questions whetheror not it's really a replica of Nigel's actual skeleton and, of course, Nigel is insistent that it actually is his skeleton.
They're all real. Norm provided them for the movie. In another scene, Nigel is even wearing a Norms T-shirt and Norm's Rare Guitars is credited at the end of the movie. You have to remember that in 1984, '59 bursts were between $5-10K, not $150K and up..