guinea pigs are €20. = The guinea pigs are
as expensive as the rabbits.The rabbits are €20. = The rabbits are as expensive as the guinea pigs.
A difference in a quality. (-) = not…
The guinea pigs are €20. = The guinea pigs aren’t as expensive as the kittens.
The kittens are €30. = The kittens aren’t as cheap as the guinea pigs.