I think Aldhick is right that monsters are overpriced. I remember it was specifically stated that High Elf Dragon Riders were, to discourage players taking too many (which would be brutal). Eventually they decided they'd been overcosted too much, and they've since dropped by 50pts. Whether they're still overcosted at all I don't know.
Even if they are more expensive than an infantry or cavalry equivalent would be, they'll always have a place. People will take them for the awesome models, and no one can concentrate the number of attacks into the small frontage that most monsters can- so a combat has to be pretty cramped before you can't get a monster in to help out.