Paint brushes, roller covers, mixing cups, trays, and other materials are consumables, built into the cost of doing a job.
Determining whether or not something is worth doing is a simple math problem. Take your rate and divide it by the cost of doing the thing, then factor in the opportunity cost of doing something you don’t like vs something you do like.
I value my time at $100/hr, as that’s what I charge in my shop and there’s always paying work to be done in the shop, so I have a constant opportunity cost. Every minute of every day, I could be making my shop rate instead of whatever I’m doing instead. So, if I decide to watch tv instead of working, I’m actively choosing to spend my time doing that at the opportunity cost of $100/hr.
If it takes me 15 mins and some solvent to clean a $15 brush I'm money and time ahead to toss it and buy new. My time is far more valuable than a paintbrush at that price.
If paintbrushes were $50 or more, the equation changes. Suddenly 15 mins of my time and $5 worth of paint thinner saved me $20 by cleaning a brush, as I’d have to work 30 mins at shop rate to cover a $50 brush.
All tools and equipment need to pay for themselves and their replacements. For every hour of work, a percentage of the rate needs to go replacing whatever tool you’re using to make the money with. Nothing lasts forever and your tools wear out. Time is limited and you make more per hour on profitable work than trying to pinch every penny out of a consumable. The cost of the replacement is built into the rate, so don’t waste time trying to save it when it’s already paid for. You can’t save your way to prosperity, you earn it.
If you’re retired and have all the time in the world, you still have a rate. It’s still the same math, you’re just paying yourself vs a customer’s money. Ask yourself if you’d rather spend your time cleaning a $15 paintbrush or spend that time fishing, watching a movie, cooking with your wife or kids or whatever. The opportunity cost may not be strictly financial.
I worked for an old car painter who would spend a dollars worth of solvent and a few minutes of his time at $100/hr shop rate cleaning out the disposable mixing cups that the paint supply gave us with the paint. Never did understand how he made any money. He did spend most of his time working on something instead of enjoying the fruits of his labor.