If you're looking for a rule of thumb to decide if something is worth the price, I have a basic method that helps me:

Don't think about the thing itself, think about what it provides to you. Say you are thinking about buying a GPU to play games on your computer. The one that seems like the best fit costs $400.

What will that GPU provide you? You will get to play the games you want for about 5 years on average before the card starts to become too old to give you solid performance anymore. Or maybe you're not too picky on framerates and settings, so it will last you 7-10 years before your next upgrade.

Consider how many hours of enjoyment you will get by playing games for the next 5, 7, 10 years. 5 hours of gaming a week, 52 weeks a year, 5 years gives you 1,300 hours of enjoyment. Now divide the original cost by that number. In our example, that's roughly $0.30 per hour of enjoyment.

Does that feel like a good price for fun to you? If you could go to a magic vending machine, pay 30 cents, and feel enjoyment for an hour, would you?

That's how I think about purchases, and it helps me quantify the value better and make purchases that are likely to be a good value to me. It also encourages me to use things a lot for a long time to get an even better amount of value out of them, which in turn is more environmentally friendly and helps me avoid the traps of rampant consumerism. Win win win.

Hope that helps!