They are far less common than pitchers and usually considerably more expensive. A majolica syrup will have a price much higher than a comparable sized pitcher by the same manufacturer.
In general there are two kinds of syrups: those that are specifically designed as syrups and those that are pitchers adapted and retrofitted as syrups.
Among the best known of those specifically designed as syrups are those made by the Etruscan Works of Pennsylvania. These designs were very popular at the time of their manufacture and continue to be popular among collectors today.
Of course other companies made these types of syrups as well.
The second type of syrup is more common than the first but every bit as collectible. These are pitchers converted to syrups by the company that potted it. These are often identical to the company's regular pitchers with one exception--they have holes near the rim to allow for the fitting of a lid. Occasionally you will see a beautiful pitcher in otherwise mint condition with two holes near the rim. These are pitchers potted as syrups that have lost their lids. They will have a lower value than either the fully fitted syrup or the comparable pitcher without the holes.
Other times pitchers would be retrofitted as syrups by a third party, sometimes with the consent of the potter, sometimes without the consent of the potter. These will often have awkward clasps or drilled holes near the handle to allow for the fitting of the pewter lid.
In all cases there should be some sort of thumb rest on the lid to allow for lifting of the lid.
If the thumb rest is missing or broken off the pewter lid it will affect the value of the piece.
Other things that will affect the value of a syrup will be staining and cracks, both of which are commonly seen on syrups.
The syrup shown above is a classic example of a stained syrup. The background was originally ivory with a green leaf on it, but molasses has stained the pottery a dark brown. It could be restored with cleaning.
Often referred to as molasses pitchers in company literature majolica syrups have retained their brilliant color and popularity 120 years after they were originally created. They truly are the royalty of majolica pitchers.