If you're not using overhead heat and you get your retics out every day for some exercise (unless they're in shed or digesting) it's likely fine for smaller ones. Adult mainlands I'd put in an 8'x30"x18" enclosure at minimum.
These things get expensive due to shipping costs. The same size tub I posted from Home Depot is also available on Amazon from a different manufacturer, with "free shipping", but it's about $200. If I got it at Home Depot I could have it shipped to my local store at no charge for pick-up for just over $40 with sales tax.
What materials were you thinking of to build the rack? And were you going to go 4 or 5 levels high?