Like most Pokmon cards, the value of Groudon-EX 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 (Primal Clash)
As far as strategy goes I would definitely try and get Dragonite ex as your active Pokemon as soon as possible, I wouldn't ever use Deafen if I could avoid it, I would always stick with Dragon Roar because with Deafen preventing your opponent from using trainers isn't that big a deal because you can always find ways to heal your Pokemon without them, if it be other Poke-Powers or Poke-Bodies or Supporter Cards
The modern chase and crossovers are the liquid entry
And it doesnt stop there, because some graded cards are now being faked too
I realize this isnt one of my normal CDS articles, but I plan on bring some of those back in the near future
You can search your library and give them lands with a few effects like Yavimaya Dryad