When I purchase something, I try to think of how often I will use it and if it seems like I will get enough use out of it, I buy it. Then I keep track of how many times I use it to see if it manages to 'pay itself off' and therefore worth its price, and worth having been purchased in the first place.
I track this through cost-per-wear and it's the price I paid, divided by how many times I use it. When do I consider something to be a good purchase? My cost-per-wear threshold is $1 per wear during its 'life span' (it might be different for everyone) - once I've used something enough to hit that target, I deem it a successful purchase!
This also tells me how long it takes me to use an item before it's worth it and through other factors (listed in my previous post about my tracking categories), tell me whether the value/quality of the item is holding up as it goes through the time it takes to meet my cost-per-wear.
The data also helps me see that I really don't need as much as I used to have because I really do only use very little of my wardrobe, constantly, and it helps me to buy less and thoroughly enjoy each item.