SampleTank 3 is more than just an update, it appears to be an entirely new product.
The bar has been raised considerably during SampleTank's long lifespan and has continued during ST3's development. They need this version to be something special, something able to compete with the top-notch libraries that have come along since ST was first introduced way back in 2001 (!). I'm guessing they're attempting to do away with every current limitation (e.g. lack of velocity layers) while greatly improving the quality of the samples themselves. I wouldn't be surprised if ST3 really blows minds when it finally sees the light of day.
The unexpectedly-long gestation may be the result of the
Duke Nukem Effect. If you remember that video game, it was a huge hit because it introduced technical improvements beyond all the competition. The company then proceeded to work on the next sequel with the goal of blowing past all the competition once again. Unfortunately, the technology was a moving target and their desire for perfection assured they would never catch up. The company went bust, the game was sold to some investors who released it in a crude, unfinished state (and it sucked).
Don't expect any huge discounts for existing customers. After all, who on this planet is
not already a customer? I expect an introductory price, perhaps $150-$200, and a regular price comparable to mid-tier Kontakt-based packages, perhaps $250-$300. That's my guess.