Pricing decisions are made by the manufacturer's Marketing. Besides the website's Upgrade pages please keep an eye on your emails for potential extra special offers (like the one in your screenshot).
@Sunny-RYU9801 ... if you want to take advantage of that offer, just hit the 'Upgrade Now' button to purchase at the upgrade offer. It is hard to believe that there would ever be a better offer to upgrade at $100 USD as against $599 USD other than if it is given away free. Presumably the offer is still available because that pop-up window still pop-ups when I opened Vegas Pro 19 just now.
For 100 USD this offer is excellent and worth your money. You get programme for special video effects (VEGAS Effects), photoshop and lightroom alternative (VEGAS Image), livestreaming app (VEGAS Stream) and audio recording-editing software (SOUND FORGE Audio Studio).