I give people quotes on custom tanks all the time, only had one taker so far. Unless it's a build in place monster (real monster - 500+) it's really tough to compete with manufactured tanks for the quality, price, warranty, and so on. If you did manage to build a 180gal tank for $600 and sell it for $1000 you're looking at probably 100 hours to do it **right**, maybe you could get that down a very long way by repeating the process, having jigs built ahead of time, etc, etc, etc. But with an average joe shop, even 40 hours is pretty optimistic (again, to do it to the level that you're comfortable selling it to someone else to put in their house), and then you're working for around $10 per hour.
I know someone will chime in and say they can do it in a weekend, and I'm sure you could, if you did it a few times before, or had a full scale professional workshop, or else did it super crappy, but factoring in design time, trips to the store for stuff, applying sealant in several coats, glueing and screwing every joint, bondo, finishing the outside, getting it to the customer's house, blah, blah, blah, 40 hours goes by FAST.
