If you need to purchase something like sustenance or pay for bills, you don't really need to decide or contemplate if you should or shouldn't spend your money on it because the answer is you should. This is for those other times.
By one definition, from my experience and past profession, I am a "security expert". Specifically, an expert on breaking / bypassing physical security measures implemented by an opposing foe or neutral target.
From this, I came up with the "The 3 Operative Laws of Physical Security". In short it theorizes that any security measure is bound by time, effort and force. That all security measures have a finite amount of time before exerted force and effort against it will break it. The trick is calculating how much time will it take to break the security using how much effort and with what force. This was the basic formula I used to see if a certain security measure was worth engaging.
How does this relate to deciding on if I should buy something?
Those 3 laws; time, force and effort can relate to earning money in the same way you break security measures. Earning money and breaking security is a process with a specific goal to reach. It's not easy and it takes time, force and effort - or in regards to earning money; time, work and effort.
That's how I decide if I should buy something that I don't absolutely need. Wether it's something frivolous, leisurely, overpriced or is of a more expensive brand / model.
I calculate in my head how much time, work and effort it would require of me to earn the money amount of the product or service I'm deciding to buy.
So if I'm in the mall and see an Armani shirt that my heart yearns for, I find out the price and imagine the actual work and effort I have already been through over the course of whatever time it took to be able to afford that dollar amount. It just takes a moment but afterwards, any emotional connection I impulsively developed with the item is severed or weakened.
This way you can see the true cost of a thing is not the monetary amount but that the force / work that you spent time and effort to obtain that monetary amount is the true cost.
Visualizing what you had to go through for a specific amount of money, even if little, can give you clarity on deciding to buy something. Because this lets you put a very personal type of valuation on any given product / service directly in relation to your "sweat, blood and tears".
What is considered expensive, valuable, worthy and useful is relative so thinking this way can help with anyone regardless of how poor or rich they are because it scales.