Regarding the segment below, 
I am use to the gift shop being before the admission booth or gate.  Anyone can enter the gift shop even if they don't pay admission to the museum.  Charging admission in this situation does not affect the quantity of gift shoppers/possible members as indicated below.
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From:   Pete & Deanna Jordon[SMTP:[log in to unmask]]

Here's you might want to consider. A good number of the people you bring
into the museum will visit your gift shop and perhaps consider buying a
membership while they are in there. If you charge an admission fee, you're
cutting this pool of museum gift shoppers and possible members down by at least a quarter.

So you'd also want to consider what you might lose in memberships and gift shop income when you're looking at what you might make by charging
admission.

Thomas E. (Pete) Jordon