Short answer: buy crates. In this case it's best to buy the Wild Hunt crates because then there's a chance you'll get the statue directly.

Crown gems are the compensation prize in crown crates. If you get a collectable item (like a costume or mount) which you've already got then you automatically get 1/3 of it's value as gems instead. Consumable items like potions and XP scrolls go into your inventory and then if you don't want them you can exchange them for gems as well.

As a result it's impossible to say exactly how much you'll have to spend to get 100 gems, because it depends on how many items from the crates you already have and your luck when opening them. At absolute maximum it should take you 25 crates, because if you get really unlucky on the other stuff you'll get 3 consumables which can be turned into 1 gem each, and that will give you the 75 you're missing.

Um, it took me 15 crates and sold some older items, so now i have that statue. not big as i thought.