For play, I use a ratio I call the Fun Factor. Essentially, that's the $/hr of fun I get out of something. So if you go see a movie and it is $10, and it is the rare money that is actually fun to watch, then the fun factor is roughly $5.
So then you apply it to other things. Sure, $2,000 is a lot to spend a mountain bike, but divided out by the number of hours I've had on it, I'm way below $5 these days, so I consider it a fair deal.
A smartphone and laptop I love is always worth it in fun factor dollars.
That said, I do take issue on the 'lasting forever' bit, because wow, stuff just isn't made to last forever anymore and nor would you want to. I think it is a clever trick that we play on ourselves, oh, "This will be the last [blank] I ever have to buy", but how often is that really the case?
I can think of a few things I have that qualify, but they are few. I bought a $100 chef knife about a decade ago, still use it daily, still love it. I don't see my Ortlieb messenger bag ever breaking down, so maybe that's another. A few hand tools, but most hand tools last forever regardless of quality.
What are yours, what have you bought that you think you might keep forever?