I found an answer. Depending on size & quantity anywhere from $5-$12 each.
https://saltyunderground.com/category/berghia-nudibranch-the-natural-choice-for-aiptasia-control
NOTE: Formerly known as
Berghia verrucicornis and recently reclassified as a new species
Aeolidiella stephanieae though currently known by its common name of Berghia nudibranch.
sea slug is typically found in shallow waters from 1 to 2 meters in depth and is known to be only from the Florida Keys region. However, due to its popularity in the American aquarium trade, most Berghia purchased are captive bred. There are other nudibranch species that also feed on Aiptasia, and which originate from all parts of the globe, however they are not commonly sold, and obtaining them can be difficult.
The adult Berghia is and is semi-translucent. Juveniles available for purchase in the aquarium trade are typically ¼ - ½ inch in length. “Adults” range in size from ½ to ¾ of an inch in length on average, with fully grown adults ranging in size from 1 to 1½ inches in length.
The ‘head’ of the Berghia has two long oral tentacles. These are used primarily as contact chemoreceptors to taste food quality. Immediately behind the two oral tentacles are the rhinophores. These are typically shorter than the oral tentacles and form a ‘V’ on the head of he nudibranch. The rhinophores are used are distant chemoreceptive sensory organs used to locate distant food sources.
Immediately behind the rhinophores are the ‘eyes’ of the Berghia, capable of very rudimentary vision and believed to be used as a means of sensing light.
Covering the ‘back’ of the Berghia, behind the rhinophores, are numerous protrusions know as Cerata. These cerata are considered to be an extension of the digestive system and form two primary functions. The first is as an aid for oxygen exchange (similar to gills). In addition, they also contain cnidosacs at the tips of the cerata.
These cnidosacs are used as a defensive mechanism and contain re-cycled nematocysts (or stinging cells) that the Berghia has acquired from the
Aiptasia they consume. The Berghia ingest immature nematocyst from the
Aiptasia then allow them to mature in these cnidosacs. The Berghia are able to encase un-discharged nematocysts within a phagosome which are then stored in these muscularized chambers, called cnidosacs, which are located at the tips of the cerata. When under threat or attack, the Berghia is capable of ejecting their cerata. (NOTE: The Berghia will not sting aggressively like the
Aiptasia anemone.)
Berghia also have been shown to harbor the Zooxanthellae, the photosynthetic symbiotic algae found in corals. While utilizing a very primitive form of symbiosis, the Berghia is able to capture zooxanthellae from the
Aiptasia. Research has shown that these symbionts continue to perform high levels of photosynthesis and remain in the Berghia’s cerata for as much as 6 days before being excreted. Regular feeding helps the Berghia maintain an active supply of zooxanthellae; however, due to the primitive form of symbiosis, the zooxanthellae are insufficient to prevent starvation of the Berghia in the absence of a readily available food source.
PS: There is much more information on sea slugs at their website.
