Museums are wanting to buy my OLD GL PLATE NEGATIVES Collection but...?

... but what should I charge for them? I've never done this before? There's 40 in all. ALL in GREAT SHAPE! One of the museums wants to buy just 2 or 3 & the other museum wants to hold a meeting to possibly buy them? On the gl plate negatives are alot of historical local famous people(s). Also I have on one of the plates is artwork from the 1800's that was related to our Olympia Brewery/Leopold Schmidts. This one is extremely RARE because it was when Prohibition hit back then & it shows the Olympia brewery Trademark of the Horseshoe on it which was also Leopold Schmidts family's coat of arms. Everyone in this area knows it belonged to him & the Oly. brewery. It has the Tumwater falls in the background & says, TUMWATER on the top of it & on the bottom it says, NATURAL MINERAL WATER. So we're pretty sure that it was during the prohibition because of no alcohol/beer being mentioned? What do I start my offers at? What do I do, should I sell them as a group, a set? Or should I sell them individually? H~E~L~P~? Thanx~....: )