Everyone who's bought a RTT has thought the same thing, I know I did. I've always been one of those people who thought "I could do this myself" and then tried. I even went through the trouble of designing my own thinking I could probably build it cheaper. However anyone who's ever started a project like this probably knows how much effort and cost it really takes, and in the long run it's usually not going to be any cheaper, and definitely not as "finished" looking as buying one off the shelf, and that's not even counting your own time and effort. If you look google DIY RTT you'll see dozens of people that have started making one, but I've seen very few that have finished, and of those that have finished, I can only think of one or two that doesn't look home built. It's very rare to be able to make something cheaper than a finished product on the market, unless you have access to a lot of cheap labor or access to skills and tools that most people don't have.
In the end you can build one yourself for personal pride and satisfaction, but don't expect to save any money. If it were that easy, most of these companies would be out of business already. I luckily realized my own limitations (or someone else realized my track record on home projects and told me not to even start) and just decided I'd rather just buy one and spend the time doing something else.