As a rule of thumb I've seen anything over 300 gallons (sometimes 500) is more cost effective to build. I think if I find someone who will build a tank that size out of glass (acrylic would be bad here for numerous reasons), and they would do it for the cost of materials (again, good chunk added to tax return), I would jump on the opportunity immediately. My grandfather is in construction, and said he can get me a powder-coated steel stand (or however many I need) for the cost of materials as well.You're right, I think it's as it gets smaller, the cost benefit shrinks. In addition, if you need any tools that you don't have (sanders, drills, specialty saws), those costs are sunk costs and make it less attractive.
I can't tell you where people said it was much less attractive to build versus buy. I recall it was in the 100-300 gallon range, but don't quote me.
Also, it depends on whether or not you value your time. I don't know many people who would give up their weekends or vacation time to build a tank for someone else for free. If it's treated as "free" (because it's a hobby), then it's a lot easier to make it look like a cost savings. OTOH, if you think your spare time is worth something ($10 / hour, $25 / hour, etc.) then it can change the results.
Personally, I would much rather buy than build. However if building is the only way, I would be willing to do it. I don't want to take a profit from this unless it's offered.