Acrylic is not that expensive, but you are paying for the acrylic and the workmanship and company costs of making them. Anything in this hobby that costs $5 to make is $40+ in stores. The cheaper stuff is either cheaper to make, or was made in China who export in here for next to nothing. But that is a whole other argument.
@J_T normally has nice ones for sale and he will be at Fragfest this year. I do not recall his being that much. I have 2 (one from JT and one DIY) and they make for a very interesting point of view on the tank. Also, some are designed with the view panel recessed into the sides, those ones are better for taking photos as when the view panel is on the bottom of the sides, light enters the edges and makes it harder to get a nice photo.
Correct: People buying acrylic here, and paying over head for a shop to work in, have higher prices. As mentioned DIY will always be cheaper. But lets break down your DIY on this viewing box.
Acrylic - Lets assume you grabbed whatever the local supplier had on the off cut shelf. You are likely in for $5-10. And probably have 1/8" or 3/16". Maybe today was your lucky day, and there was some 1/4" available.
Solvent - Weldon is likely what you will use; #3, and #4 come in small cans, and will run you about 15 bucks. But now you need something to dispense this with. Add another $10. Or, maybe you went with Weldon 16. Syrup thick, messy to use, and not really meant for this kind of work. But its DIY, and will "work", but you will give up the clean look, add $10.
At this point you should have a box that is water tight (maybe?) that will work. Time, 1 hour, and spent $15 - $35 to build your viewing box.
I understand the feeling of doing something yourself. But lets compare that to what you get from a store bought unit.
Buying from a store means you are getting something imported (likely). Thus you are getting thin material (don't drop it ever). But it will be polished, and look nice. Part of the cost associated with this piece, is that the owner of the store is paying for a building, staff, and all the other bills that come with having a business.
Now, time to toot my horn...
My 6" x 6" viewing box is made with 1/4" material, polished edges, recessed clear panel, and will cost you $30. I will have these at the show. For sure I will have my Black acrylic ones, and depending on time, I may have some other colour choices.
If you would like a custom colour for pick up at the show, drop me an email, and we can work it out.