I've been searching high and low for a solid second-hand ergonomic chair around the $300-500 mark. Not sure what the go is, but haven't even come close to finding anything half decent.
I am now considering purchasing new around the same mark, but obviously there would be decrease in quality for the price,
Ive narrowed it down to two but open to suggestions and feedback;
Sihoo M57 - $400
https://sihoo.com.au/products/sihoo-ergonomic-office-chair-c…
Ergohuman Fit IOO (Ive been waiting for a sale but nothing as of yet) $530
https://www.templeandwebster.com.au/Ergohuman-Fit-High-Back-…
https://www.officeworks.com.au/shop/officeworks/p/haworth-al…
This one for $500 is quite good. But like everything else, each person has their own preferences and you should go try it out before actually buying it.