With the traditional enclosed rack design using plastic, for larger tubs it means alot of plastic material and so the cost reflects that. Just can't have a very inexpensive plastic rack for large tubs. The racks made for ball pythons are a fairly reasonable middle ground, and if that size is acceptable for your needs, not a bad deal at all.
The open racks sold by mfgrs are expensive, period, due to the superior materials and exacting specifications. Great products though.
So, the least expensive is an open-rack design you make with less-expensive materials at home. For example, with melamine shelves and side supports made from perhaps 1x6 melamine, 3/4 x 6-inch-wide plywood strips or 2x4s, two per side run vertically to which the shelves are attached. Cost of material-wise, can't beat it. Boas are big, the gap between shelf and tub rim doesn't have to be too precise, especially if you use lids on the tubs. A cabinet shop might do it for quite a low price if you ask around. Beware of the weight of wood racks though.
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Mark