Experience. You'd might as well resign yourself to the fact that good brushes are never cheap. When I was contracting, the local Sherwin Williams had some really good seconds in the back, bad places in the handle or something cosmetic, that sold for $2 per inch($6 for a 3 inch brush, etc) but that's been over 10 years ago. Also mistinted paint for 2 bucks a gallon. You could find really high quality paint and tint it some more to darken, or else mix with white to lighten it. I don't know if they ever let the general public in on that stuff, but if you ask, they can only say no. You could also ask which brushes painters buy. An all purpose type store, such as Lowes or Sears is not a good place to learn stuff, either.
I am hoping to benefit from someone else's experience rather than keep buying brushes until I hit the jackpot -- I will ask next time I am at at the local store. It is rather hard to force myself to pay more for a paint brush when a cheaper one is right next to it I must admit, but enough falling apart brushes and spattered paint helps motivate.
I've been very satisfied with Purdy brand brushes. They are not cheap, but the quality is outstanding. In my experience, buying a great quality expensive brush is cheaper in the long run than buying many cheap brushes. Remember to buy a polyester brush for latex and water-based paints and a natural bristle brush for varnish and oil-based paints.
I've watched a lot of makeup videos that use paint brushes for makeup. Depending on what you're painting, if it's artwork (sorry if you mentioned it and I missed it) then the opposite could be true. Instead of hardware stores, the next time you're in Target or Walmart, E.L.F. brand has really cheap ($1-$5) but good makeup brushes that might help/work for what you want to do. Sonia Kashuk brushes ($5-$10) are really soft and nice as well, horse hair I think? I hope you don't think I'm cuckoo, I figured if painting brushes can be used for makeup, then makeup brushes "could" be used for painting
