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I do not consider gear upgrades to be financial investments in this game. Because the gold and gear you farm is minimum compared to what you can make from the Trading Post, I would never spend more than a fraction of my gold on upgrades.
I actually have a similar post on my Diablo 3 website, where I will discuss setting up a "25% maximum spending rule" in that game. I guess the same can be applied here as well.
When upgrading your character, never spend more than 25% of your total gold on each item.
Again, this is because the items themselves do not add enough gold earning potential to consider them an investment.
An investment brings you in more gold (hopefully) than what you initially put forward. If you are going to spend more than 25% of your gold on an investment, then you can start deciding if the risk is worth the reward. That's a simple matter of research.
For example, you could look up a few disciplines on GW2Spidy.com and see if there are enough opportunities to make gold crafting to justify investing gold into the discipline.
So if I have 100 gold, and I think investing 50 gold into tailoring is worth it because I'll make X gold every day for the rest of my existence as a tailor, then hell yeah it's worth it! Sure, I spent 50% of my gold, but I know that I will slowly get it back.
But I'm not going to spend half my money on a shiny new sword, especially since I can only salvage it when I go to upgrade (unlike in Diablo 3 where your dusty old sword can be resold as a down payment on your next one).
Remember to treat investments differently from upgrades!