When it comes to cost.. improvise, people!
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A plastic tupperware from the dollar store with needlepoint canvas will cost about 2$ (for two bowls) and makes great water bowls, usually much deeper than any specialty ones you can buy from the pet store. Shells can be found at craft stores, dollar stores, even online. At pet stores shells can be 4$ a pop. Try playsand instead of specialty critter sands or substrates, at 3$ for 25kg, it can easily fill up a 10 gallon with some to spare. Small shower caddies, suction cups, plastic plants, terra cotta pots and dishes, river rocks and even shells can all be found at the dollar store. Nets for second levels can be found at craft stores.. the list goes on. Temp/humidity gauges can be found better and cheaper at hardware stores.
But in a nutshell, if you're trying to save money, don't buy from pet stores!
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You can find a lot of the stuff you need (sometimes better stuff) elsewhere.