Three reasons . . .
First, the whole point of a gift shop is the make money for the business, not cost money to the business.
Second, whatever monsters you create and/or acquire need to be used in the Haunt first. Now as you pick up stuff along the way, and get extras, cast-offs, and discards, then eventually you can massage all that into a really cool museum type thing.
Third, space is precious and dear, and any space occupied by something you can't sell is revenues lost. Look at most museum gift shops. They are usually beautifully appointed, and in many ways are almost as compelling to the eye as the exhibit was itself, and yet every single thing in the whole place is for sale.
Now, if you have a serious nut for making a monster museum, here's a cheap and effective way to do it. First, do some dumpster diving and find a store that is getting rid of some mannequins, or whatnot. Next, make the mannequins up as classic Hollywood monsters, including some modern classics. Find the masks cheap where you can. Basic Dracula or Frankenstein or Freddie masks should be fairly easy to source cheaply. Next, make the mannequins up, only they are wearing your event t-shirts, and a hanger with the t-shirts are right next to them. Then, add in whatever prop weapons they would have, or Dracula gets a cape over his t-shirt, and so on. Pick up the pants from Good Will.
Also, you had talked photo opportunities. There are two ways to do this. First, have your icon characters who are working the line available for quick camera phone snaps. Those moments are always precious and immediately get posted on facebook, tweeted, and so on, which is free advertising. This pics, therefore, should always be free. Second, have one nicely appointed scene set up in one corner of the gift shop or nearby where people can either pay a buck to take a pic with their camera phone, or $5 for a nice pic in taken, and then put into a nice frame or mat with your haunt's name and website info set up in the corner, like people do at Six Flags and so on.
Now, as to only having stuff for sale in your gift shop, and to your question of what else would be feasible, there are a lot of things you can do. In addition to shirts, you can also sell other kinds of swag with your info on it, but that may take doing it subsequent seasons to even out the cost. If you go to places like US Toys and Oriental Trading Company, you can order all sorts of neat stuff like glow in the dark spider rings, glow in the dark rubber balls, rubber eye balls, and whatnot, all for mere pennies a piece, and then turn around and sell them for $0.50 or whathaveyou and have a marvelous little souvenir for the kiddos to take home. You can sell drinks and bags of chips on the way out. Being scared might have made them thirsty or hungry. There are any number of things you can do to start with.
Now, I wouldn't go too crazy with it, since you don't know what your first year is going to be like and how many people you are going to have to sell to, but would be a good way to test the waters rather cheaply.
Those are some initial thoughts.
When I put my gift shop together, I'm going to be incorporating some rock music into it, and I think it would be cool to sell some band swag of these different bands, or carry CD's or music downloads. If you have some classic monsters, then perhaps some DVD's of the classic monster movies.
There's lots of potential there, but you don't want to blow $2000 on a fancy monster museum that only brings in $200 the first year if your lucky, especially if all those monsters are needed thru-out the rest of the haunt.