Well, actually nowadays, 3d glasses are very expensive and they are very popular with many people, especially young people. And first, you should know that the 3D effect relies on your two eyes each seeing a slightly different image, in which your brain then combines into a three dimensional picture. Because of their special functions, they are more attractive than the regular glasses. So when you have worn a pair of 3d glasses while watching 3d movie, it will make you feel that you are just in the real phenomenon. Anyway, it can be amazing. And because of this special function, 3d glasses should be made with special material too. And that is why they are so expensive. But in my opinion, you can just buy ones and have a try.
Different production of raw material of 3D glasses, different price. The lowest prices one is made by red, blue and green color glasses to form the complementary colors, just made by transparent plastic material. The higher prices one is made by resin with better filtering rate. The best one is the one which used in cinema, it would bring enjoyable feeling for you to experience the stereo effect and vividly feature. You will get what you pay for.
Ok, it seems that you are wondering why 3d glasses cost you so much money. Anyway, on an economic basis, the answer could not be explained easily. But as you can see the reason why they are so costly is that they are primarily made for rich people and the upper society, which could afford more expensive products, which make those 3d glasses quite expensive, but with the passage of time, they tend to be cheap, that is a social trend. Try to look at Amazon, where you could see some 3d glasses at reasonable prices.