Commodity price falls below 0, becomes $99,999
When a commodity price falls below $0, it becomes $99,999 for 4-5 seconds. Then it bounces back to a low positive value around $10-15. In five minutes it drops below 0 again, etc. until the price trend reverses and becomes increasing. You cannot trade to drive the price below 0 yourself - it is caused by a steeply declining trend driven by the game itself, and the current price has to be around $10-15 for it to drop below 0.
This looks like a trivial programming mistake by the developers or a glitch intentionally left for attentive players to exploit.
Silk is $99,999 on the screenshot.