I'm with everybody else in that the price of those terrain pieces does not seem unreasonable, although I would never pay that.
The problem with terrain is the time involved in building and finishing a piece. So a scratch built piece that is hand painted (depending of course on materials, quality, etc.) could easily take enough time to make to justify the above-referenced price. Those pieces look nice enough to justify those prices.
I tend to make my own terrain, and I make a crap-load of terrain. I also prefer to make my terrain on the cheap, cheap, cheap. That said, I also use some laser cut terrain kits.
Terrain kits are always going to run more than scratch building terrain, especially if you are making a
lot of terrain. The question is one of time, money, and desired appearance.
If you have a lot more money than time, buying finished pieces is a good way to go. If you have more money than time, buying kits can be a good option. They generally go together faster than scratch built terrain, and there's less chance of screwing something up. If you have more time than money, scratch building is probably the way to go.
Now, in terms of final appearance, if you are okay with something that is pretty good looking, you can make some decent-looking terrain with minimal skill, especially if you work on projects that forgive mistakes. This is one reason I love making ruins. It is hard to screw up ruins. If you want something that looks awesome, you may be happier buying quality kits or finished pieces.
Personally, I tend to reach for a kit when scratch building the relevant piece would be a pain in the
. For someone that makes a lot of terrain, I HATE measuring and cutting. I hate it with a passion. I do not like cutting out walls, making sure everything is the same size, measuring out angles, bla bla bla. I hate that crap. For me, it sucks all the fun out of a project.
So if I want a clean, whole, undamaged building that is a complex or annoying shape, I will often look for a good laser cut alternative. I also buy kits that save a great deal of time, or give me something that I couldn't easily do scratch building. For example, I recently got the Gamecraft Outhouses kit. I could have easily scratch built outhouses, but for 15 dollars I got 8 outhouses that looked pretty good, went together in a few hours, and had little laser cut moons and stars in the doors (something I would have been hard pressed to do on my own). So for 15 dollars I did in a few hours what would probably have taken me twice that or more.
When it comes to hills, rocks, and natural terrain, I always scratch build. The materials are dirt cheap, it is hard to screw up, there's little need for precise measurements, and required skill level is low. I usually scratch build ruins. I scratch build simple structures, like square houses with few windows.
All in all, it depends on what you want. If you want to fill a table and you aren't looking for the best-looking terrain around, scratch building is good to try first. You can grab some tutorials from sites like Terra Genesis, or DakkaDakka, and you can see if you like what you wind up with. The investment will be comparatively minimal, you could very well be happy with what you come up with, and you may find that you very much enjoy making your own terrain.
The key to scratch building terrain is to make sure that the mundane nature of whatever you are working with is hard to identify. If you nail that, you can knock out a lot of decent-looking pieces very quickly with little investment. And the best part about scratch building terrain is that the more you do, the cheaper it is. When you can buy your materials in bulk, cost starts to go way down. Bag of play sand, gallon jug of glue, 8x4 sheets of plasticard, 8x4 sheets of hardboard, 8x4 panels of board insulation, homemade molds, etc.
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