Welcome to TONMO, the premier cephalopod interest community, and birthplace of #WorldOctopusDay and #CephalopodAwarenessDays. Founded in 2000, we are a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up. You can also become a Supporter for just $50/year to remove all ads and enjoy other perks. Follow us on Twitter for more cephy goodness.
IMO you won't save any money building a tank unless you've got something awfully big (200+ gallons?) in mind.
Find a nice used tank. If you watch eBay for a month or two you'll probably find something within a few hours' drive, and also check out the classified pages on Reef Central, but beware things there get taken real quick. You might also peruse your local craigslist.
The copper is a good point, but if used is the only way it's affordable its a necessary risk. I don't think most test kits (designed for dosing for fish) have a low enough threshold to detect levels harmful to cephs, so your impression of the current owner is probably your most valuable ally.