Over-simplification
You're over-simplifying it, Az. It's like saying that paying a laundry service is awfully expensive, given that they just give you your own clothes back a day later. But addressing this will give me a chance to reveal what goes into the design and production of a Roomettes kit.
First of all, to make a comparable item yourself, you'll need CAD software and graphic design software, as well as the skills necessary to use them. You'll also need to have experience using them together, because sizes change when you import one file into another program, and then export another file for printing. Ask me how I know.
So you've measured the building and designed the basic room shapes, and you've printed out a test blank and cut and folded it, and you find out all the ways it doesn't fit the building, and that the window opening is .5 mm off horizontally, and now you can make changes to your CAD drawing and do it all over again.
Now you are probably ready to paint the room. You will need to find appropriate images, at an appropriate angle, with the appropriate lighting for the era that you are recreating. The last room I designed was a bedroom with over 70 separate elements in it. Go into a retail store with your camera and take pictures of everything. Get permission first or be ready to be asked to leave the store.
Don't forget to design other details like shelves and counters. They add to the 3D effect.
Next you'll need a decent printer or else you'll have to run down to your local print shop to have it printed on stock heavy enough that it won't warp when you glue it. That will cost you about four bucks.
When you get that home you'll have to cut and score it to fold it into a room with tabs for gluing. As I have to hand-cut each prototype model at least once - and often twice after making revisions - I can tell you that it's more than tedious. It's my least favourite part of the job. Don't cut in the spot where you meant to score it, or you'll be going back to the print shop. It sure would be nice to have an elaborate and expensive tool that would do the job for you quickly and accurately...
If you have a laser in your workshop you can get it to cut the room for you. It will do a perfect job. It will cauterize the paper at the fold lines and give you a really nice bend. The edges will be straight and the corners will be square. But the set-up and testing for this takes 10 times longer than cutting it by hand, so unless you're cutting out a few dozen of the same pattern it's not worth it. Also, you have to figure out a way to tell the laser where the image is on the paper. That's a whole other process that only works for mass production.
So now you're ready for LEDs. I'll assume you're comparing apples to apples here, so your interior lighting will need to be compatible with Woodland Scenics' Just Plug system. You can buy a package of two Just Plug LEDs with JST plugs on them for $10.99. That's an LED, wire, resistor, and plug. Considering that a Roomettes kit with two rooms and a similar LED harness is $9 to $11, I think most people can see the relative value. Compare the two products below.
So yeah, if I just threw a piece of cardboard and an LED in a plastic bag that product might seem costly. But to be fair there's far more to it than that.
I won't even get into how many hours I spend building the display models to help inspire people to try lighting some of their structures. Luckily that's one of the most satisfying parts of my job.