Charge them a fair rate, as patorak suggested, so you can do more of this stuff for other museums.  Showing it on their website should be okay; I wouldn't charge more for that.   Maybe you could work out a deal whereby you sell copies of the models through their online museum store.  I've seen 3d exhibits of this kind at Fort Ancient and Cahokia and other archaeological sites and would have bought the models had they been available.