Ergonomic upgrades under $100: spend where you feel it daily.
The highest-value upgrade is usually the one that removes a recurring awkward position. Start with the problem you notice by mid-afternoon, not a generic shopping list.
Fix screen height first
If you use a laptop as your main screen, a stand plus an external keyboard is often more meaningful than a desk accessory. If you already have a monitor, adjust its position before replacing anything.
Support the chair you own
Before buying a new chair, check its seat height, back position, and whether your feet are supported. A simple footrest can help if lowering the chair would make the desk too high. Avoid treating any one item as a universal cure.
Make movement easy
A timer, water across the room, or a clear floor path can prompt more position changes than expensive equipment. Comfort comes from setup and movement together.