Is Bolas2 worth 850?

Yes, he is worth the 850 crystal cost.

However, whether or not you need him is dependent on if you have Bolas 1. The two are entirely interchangeable. Fairly similar mana gains though Bolas 1 has a slightly better curve, and while Bolas 2 has a more varied and honestly more useful array of Planeswalker abilities, the real kicker is his second ult, whereas Bolas 1's strength is his first and his other two abilities are situationally decent.

Taking away StV's absurd ramp capabilities (or some lucky cascades), getting to Bolas 2's second ult will take time. Time which can be afforded in numerous PvE fights but much less so in PvP. He's heavy on control but you need the time to reach that control. Bolas 1 takes far less time to reach and then continually utilize his first ability.

Both God-Pharaoh and Ravager are worth 850 crystals. Do you need both? No. If you have one, you can table the other until such time as you have a surplus of crystals (and runes) to throw at a Planeswalker that serves many of the same purposes with a different flavor.

I have him at about 40 and am leaving him there.
Getting him to +3 Black was important for me, but too expensive to level the rest.
But I'm using him for "Win with < 20 life" things and his level of control and ability to hurt _himself_ with his 1st on the flip side make him perfect.