UPDATE designs
SET prices = '{ "at": 507, "ch": 751, "de": 447 }'
WHERE prices IS NULL;

SELECT * FROM designs
WHERE prices = '{ "at": 507, "ch": 751, "de": 447 }';

ERROR:  operator does not exist: json = unknown

SELECT * FROM designs WHERE prices::TEXT = '{ "x": 3 }';

SELECT * FROM designs WHERE prices::TEXT = '{ "x": 3 }'::JSON::TEXT;