Here's the problem: cheap office chairs are not ergonomic. If you're not willing to pay for two-axis adjustable armrests, adjustable seat pan depth, adjustable recline, proper padding, and lumbar support, then you don't have an ergonomic chair.
I'd recommend buying an expensive ergonomic chair from Craigslist or an office furniture liquidation company. You can pick up a great chair at 1/3 the retail price. You should be able to find a Steelcase Leap V2 for $200-300. Then make sure you download the user manual off the web so you know how to configure it correctly.
If a cheap office chair happens to fit your body shape and preferences, it's ergonomic for you, even without adjustments.
It's just a crap shoot as to whether or not the fixed adjustments of the cheap chair will match the settings you would select on an adjustable chair, and if the materials are fine for your size and weight. If they do, hey you got a great deal on a decent chair specifically for you.
I'm a pretty light guy, and I think I've only owned two <$150 chairs since 2002.
The other problem is that cheap chairs will often feel pretty good when you first sit in them. But because they're made with cheap, low-density foam, they quickly pancake, and lose any semblance of comfort and ergonomics they might have had.
I used to do the $100 chair every year or two thing too. Then I bought a Steelcase Leap (for considerably more than $100), and while it's only been a year or so, it looks and feels as good as when I bought it. I would have loved to have found one for $200-300, but even buying new it was well worth it.
You can pick up a great chair at 1/3 the retail price
Pretty much this. I picked up a used Steelcase chair (don't remember the model, but it was listed for $800 new when I checked) for $65 at an office furniture liquidation company and it's the best chair I've ever owned.
Which the Aeron doesn't have. I don't think any Herman Miller chairs do, actually; Mirra has an option to not dig into the calves quite so hard, but that's it.
I'd recommend buying an expensive ergonomic chair from Craigslist or an office furniture liquidation company. You can pick up a great chair at 1/3 the retail price. You should be able to find a Steelcase Leap V2 for $200-300. Then make sure you download the user manual off the web so you know how to configure it correctly.