The tanks themselves can be one of the cheapest parts if you get a good deal. Rock & Sand is probably the most expensive part outside of livestock.
Keep in mind a bigger tank needs bigger lights, filtration, pumps, powerheads etc not to mention sumps & plumbing.
When I built my first 29 last spring, I bought equipment that was either way oversized or modular to accommodate upgrades. My next upgrade will be a 120 Gal
to an ultimate 240. I'm gradually adding pieces to get closer to this goal without any major one time expense or waste.
It's like saving but with corals growing, having a tank running and no pile of gear waiting to be used.
I've recently plumbed a 32 Gal
Brute reservoir into my sump/Fuge to handle the 100 lbs of rock I'll be adding before I go 120. Then adding an MP40 & another Radion. This way when I get my 120, I'll only need sand & water.
After that point, 100 more lbs of rock into the Brute to wait until the 220. Then it's another MP40, another Radion & an upgraded skimmer at this point (current skimmer then goes to frag tank ill be setting up) then I'll be ready to rock.