Economies of scale play a role here. Since you're unlikely to be able to buy a single strip of acrylic and plywood of the right size at a good price, building a single one is nearly as expensive as building 5 or 6 of them (not taking the price for th LEDs into account
The writeup lists rough prices for the materials. All in all, it should be less than $20 for one fixture if you buy 24"x48" sheets of plywood and acrylic and just build as many fixtures as you can make out of that. That's also assuming you've got the power supply already taken care of - needing only one transformer for multiple lights keeps the cost down.