How do you decide whether a purchase is worth it?

Rules for buying anything (cost per week, would I buy it alone on earth, the thing or the idea).

Axis: spend freely on what adds value … minimise spending

Answers by date: 4 records in 4 sources, newest first; the answers above are the current ones. The advice was refined (2018), never reversed, in these records.

date	who	answer	source
2020-06-07	Ali	Ali judges a keyboard's price by use: $500 is where it must be very compelling, but $250 is fine since his AirPods cost that and he uses his keyboard ten times as often.	My Favorite Mechanical Keyboard - IQUnix F96 Coral 00:12:53
2018-06-09	Ali	Ali would still probably buy the iPad Pro himself: spreading its £300 extra cost over the three years he expects to use it, about 150 weeks, comes to about £2 a week.	iPad Pro vs iPad (2018) comparison for college / university students 00:05:52
2018-05-21	Ali	Ali used to feel guilty about buying a £2.50 latte, and still does a little, but thinks in exam term it is worth spending it to increase his happiness and enjoyment.	Study with me + More revision tips - Cambridge University medical student VLOG #34 00:02:26
2018-04-14	Ali	Ali spends a lot on camera equipment that reduces the friction of sitting down to make a video: a tripod is worth £50 rather than stacking books to hold the camera.	Last Minute Study Tips, Flashcards, Essays & Maths - Exam Revision Q&A with Cambridge students 00:15:14