Condition and rarity are everything when you try to evaluate the price of an antique.
This is from Enfield Snider.com
There were five patterns of Snider produced, the following were standard 3 band Enfield conversions:
MkI Designed for the original Pottet cased round - The breech has a rounded rebate for case rim.
MkI* The MkI's were altered to use Boxer case ie the breech rebate altered to square.
MkII* As per MkI* but newly built that way rather than altered from MkI.
MkII** Purpose built, the breech block design was changed to strengthen the system.
MkIII Purpose built, with steel barrel and locking lug mechanism for securing breech which replaced the latching pin of earlier models.
SNIDER MK II** LONG RIFLE
This Example Lock Plate Date: 18
Manufacturer (s): B.S.A. L.S.A., Enfield
Bands: 3
Furniture: Brass
Hammer Face: Cupped
Sights: 950 yards
Barrel Length: 36.5"
Overall Length: 54.25"
Weight: Lbs
Bore: .577
Rifling: 3 groove
Notes:
This is the most commonly encountered Snider rifle and was improved in five ways from the MKI* and the MkII*:
1. This was the fist Snider to have a cupped hammer
2. The firing pin nipple was slightly shortened
3. Reshaping of the extractor
4. breach block face enlarged
5. shoe altered in respect of he above alteration
If the original cleaning rod is sill present this will have a squared jag, with a reduced diameter - most however were replaced with later roundheads, as with the original P53.
The breach is simply marked with II**, denoting the arm mark...Finally no MkII model rifles were produced.
Pattern approval date: December 1866
http://www.enfield-snider.com/Snider Mk II Star Star Long Rifle.htm