Last year during lockdown I had some time and had a busted focus on a manual lens and some dust inside. I figured it was busted anyway so it wouldn't be much of a lose if I tried to fix the focus and clean everything.
So I disassembled everything being careful not to break, lose anything or put anything back together in the wrong order. It worked great and I actually fixed two more lenses the same way and then even graduated to fixing an old DSLR.
So I guess if you don't want to spend money, you can try and clean it yourself.