IMo the cost debate is pretty much moot for several reasons.
1) When you DIY, saving money is sometimes the reason, but for most of us it's to learn lessons and experience the satisfaction of creating something with your own two hands. Cost doesn't become irrelevant but value takes on a whole new meaning.
2) You can't exactly go to a big box store and pick up a custom 4000 gallon tank, so the $1/gal benchmark is sort of meaningless.
3) The whole community is learning from the successes and setbacks here.
4) The time you spend DIY is (often) time you choose to spend. DIY is a luxury activity in most cases, and you can't very well bill out doing work instead of watching TV at $50/hr.
Anyway, for some people who only DIY to save the cash, sure, but MOST DIY projects cost at least as much or more as a commercial solution, but they wind up nicer, or more customized, or better suiting the taste of their creator.
Just my $.02