Hello Maondaer !
Welcome back to the nostalgie game !
I done same as you, back to this unique game 
I've done the investigation of prices (Actually on market) :
WHITES:
Necrid Horseman : 1K, Juggernaut : 1K, Kvelduf : 1K, Roaring Ether : 1K, Seer : 1K
PURPLES:
Flowstone Elemental : 3K, Freezie : 3K, Burning Titan : 2-4K, Ooze : 1-5K
GOLDS:
Flaming Djinn : 14K, Dagnar Stonepate : 5-12K, Lich : 3-5K, Zhed Shadowhoof : 3-5K.
Hope this helps to you, deciding what to sell or to do with then. If you wanna ask me something, my IG : tharone
its plassure.