Spider is easy mode, you can set it at a champ and go to sleep. Tanky AF.
Dragon does phat DPS. Steed pairs with it awesomely, due to Paralytic.
Crabs are new and still being worked out (how best to run them, etc).
Fully leveled/rolled over dragon will run you like 14m+
A Spider will run you 90m +/-
Crab eggs are selling for like 7-8m (and dropping). Some of the relics take Powerscrolls, so 24m+etc, some are drops.
When your dragon reaches level 7, it will continue gaining XP, and once finishing another level, it will "roll over", this gives it a new roll on it's stats, which you can accept or decline. It also lowers the taming required by .5. At max rolls, it will get down to 116 taming required, making it much easier to control. With 115 Taming/115 lore, you can control a max roll over dragon quite well. When they're not rolled over, they won't listen and you'll have to spam commands.