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Many years ago, I bought a cheap disposable calculator sold under Dollar Tree's "Jot" brand, & I chose it because it was the first dollar-budget calculator I've ever seen that isn't a simple 4-function that can't correctly answer "3 + 4 × 5".
It's the Scenery Electronics Limited SS-6618. Description and manual: https://externaldocuments.com/articles/calculators/dollar-tree-jot-scientific-calculator-10-digits/dollar-tree-scientific-calculator-scenery-electronics-limited-ss-6618-manual/
Today, I replaced its dead battery. It lives and works again! But I discovered the back plastic has to press down on the battery to keep it on the contacts.
Parts consumed: LR-1130 battery (1), $0.40.
Tools required: Driver and bit from an electronics toolkit, $25.00 in my case.
Labor expended: 5 minutes (0.083 hours), $4.17.
Total cost: $25.00 initial investment + $4.57 parts and labor.
Comparison with replacement cost: $1.00 if I'm lucky, $10.00 if not.
Evaluation: Worth it! Not just the satisfaction of maintaining something not designed to be maintained, but of successfully refusing to dispose of something literally made to be disposed.