Do you need expensive brushes?? My answer is a definite, "No!" I mean if you want beautiful, spendy brushes then go get 'em! But if you are wanting to improve your makeup application by adding brushes or you need to replace old brushes, etc. You can find some A-mazing brushes that are super affordable.
Personally I love e.l.f. studio brushes and EcoTools brushes but I have also recently ventured into buying cheap brushes from Amazon after researching reviews and I have been quite happy with those purchases too.
Brushes I recommend: e.l.f. studio line, e.l.f. professional eye shadow brushes (these are super cheap and are also good as long as you just stick with the eyeshadow brushes, the face brushes in this line are not good), any EcoTools brushes, and Amazon brushes with great reviews. Link to my black set here and similar to my gold set here (the set I purchased is no longer available but this is from the same seller and looks identical).
I could make this a humongous post with a review of all of my brushes and which brushes I think are most essential, but I am trying not to do crazy, long, and hard to organize posts anymore. Trust me, this is so much easier to read!