Like most Pokmon cards, the value of Giratina V depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Lost Origin)
D - Force: Not something I personally have much experience with as Ive never run it, but I have seen it talked about as a one-of tech card
Being so hard to one shot you typically give the blue player the chance to untap with Tamiyo still alive which means even if you got her low there is a high chance of some Confluence, Command, Cyclonic Rift or other tedium that is going to ensure she lives longer still
If there is a Nidorina on your Bench, this attack does 40 more damage
You should have some plays that you can make early in the game, but there's no mystical number for how many it should be