Generally speaking, doing
anything yourself saves you money as opposed to paying somebody else to do it for you. Big tanks are no exception. The thing is, you need to be able to do it, and you need to shop smart for the materials. Personally, I won't pay someone to do anything I can do...or think I can do.
I draw the line somewhere before getting to the point of DIY surgery, dentistry, etc. but otherwise...let me at it. Don't necessarily expect to love every aspect of
doing it, but it's great to
have done it.
Some or even much of the money you spend on projects like this will go towards tools which of course you retain for future use. There is also the difficult-to-quantify but very real sense of satisfaction that comes from doing something for yourself. And of course, you get to create something that is custom-designed for your desires and requirements rather than some off-the-shelf version that might not be ideal.
Do lots of research, take your time, don't cut corners in either cost or effort, and always measure twice and then cut once.

There are lots of terrific threads on MFK and elsewhere that detail exactly how to go about it the correct way, and there are even more that illustrate how not to do it by describing what went wrong.