Dividing a purchase price by a large number of future uses can make almost anything look inexpensive. A cost-per-use estimate is easier to judge when its assumed number of uses is visible.
Keep observed use separate from hoped-for use
For a hypothetical item costing 120, ten uses give a purchase cost of twelve per use; sixty uses give two. Those calculations are correct, but the second does not establish that sixty uses will happen.
If you already own the item, record completed uses separately from a forecast. Additional costs, such as maintenance or consumables, need their own treatment if you want a broader comparison.
Test a modest range
Write down a low-use and a higher-use scenario and explain why each seems plausible. Avoid choosing only the number that makes the purchase look attractive. The exercise is a way to examine an assumption, not a universal rule for deciding what to buy.
When reading a money book, notice whether the author has quietly shifted from actual use to expected use. A clear label keeps the example honest and lets you compare its circumstances with your own without mistaking a possible future average for a saving already achieved.