Good chairs aren't cheap. We're talking $800 for a high quality, new, warrantied, chair from a good supplier like Steelcase or Herman Miller.
But those same chairs, in great condition, on the used market generally can be had for $400 to $500. $400 isn't that expensive for a chair when you think about it.
Let's assume a quality engineer costs $100,000 per year (pay, benefits, taxes, and insurance) for a business. With 260 working days in a year (5 days per week, 52 weeks per year), that's $385 per day that the business is paying for the engineer's time.
What does that mean for chairs? It means that over the course of a 3 year span where a quality engineer works for you, it's cheaper to buy a $400 chair than it is to have that engineer miss 1/3 of one day of work (that's 3 hours) per year.
Or think of it this way, if having a good $400 chair makes your engineer 0.13% more efficient over 3 years, the chair pays for itself. That's 3 minutes more output per week! 3 minutes!
Or, brand new $800 chairs mean you need to get 6 minutes more work out of your 3 year engineer per week. Still not crazy, just over 1 minute per day more work to break even.