Ermanno Nason is a highly respected glassblower. He worked for Cenedese 1964-71 where he was master blower and designer, along with Da Ros. So everything about your piece so far points to it being authentic, but we'd need to see the piece before anybody could make further comments. Without seeing it, the range of prices make an estimate meaningless.
Lucky man, and it's time you placed it somewhere other than "a back room."