It costs 475 gold.
A green ward costs 75 gold. Unless you purchase 6 wards you'd never get your moneys' worth. Plus even unupgraded, you get free wards anyways.
When and why would you upgrade the yellow Trinket before you're maxed out?
I do it usually after my first 2 or 3 big ticket items, around the time I get my enchantment I get on my boots.
Reason being, at around that time of the game, I want to place wards like mad, as I usually try to have alot of wards out. However, I'm usually past the farming stage, so that little bit of investment for the additional vision without having to B every 2-3 minutes is VERY nice. Especially around drag/baron.
Also, if you aren't placing more than 6 wards, you are doing something wrong. I don't care who you are, if the game lasts longer than 20 minutes, you should feel bad for not placing that many wards.
As far as the pink upgrade, I get it when I need a pink in teamfights. Usually against champs like twitch or akali, but also against champs like Eve.
Usually I upgrade the trinket when I am already full build.
Once you get to mid game (about 15-20 minutes in most cases), every role except ADC should be switching over to red trinket. You could theoretically be leaving your base with 25 wards if you really wanted them, but you're limited to only one pink per player, and those things are expensive. Because of that, it's more slot efficient to have vision denial in your trinket slot.
The reason ADCs keep their yellow trinket is because it prevents the enemy team from bush-juking them. It's not uncommon for ADCs to pick up a blue trinket for the same reason.
The only time you want yellow trinket in the late game is if the enemy team starts buying multiple blue trinkets, because then they can't clear out your trinket wards and you can starve out their pinks.
So, to answer your question - almost never.
Which role are you again?
