1 crate key = 1 platinum, but only work to open a LOCKED CHEST,
that mean: to open a

normal cryogen locked u need: 5 platinum or 5 CRATE KEY
Large cryogen locked u need: 10 platinum or 10 CRATE KEY
Massive cryogen locked u need: 15 platinum or 15 crate key

And especial locked event i guess is the same value.