As already stated, you have a 1st class civilian issue cross of the Order of St. Stanislaus issued to a non-Christian by maker EDUARD, in 56 zolotniks (14kt gold) and later converted to a brooch.
It is appears to be authentic, but it is hard to say for sure without personal examination.
As a general rule, to figure out which class it is, I use the following table which I compiled myself from examples seen in museum and private collections and on various auctions:
I class - 55-65 mm ACROSS or so or so i.e. 5x mm or larger
II class - 45-48 mm or so, i.e. 4x mm
III class - 35-38 mm, i.e. 3x mm
Non-Christian issued Russian awards are quite rare and are in high demand. I recommend to have the back pin professionally removed to return the order to its original state. The loss of a loop is unfortunate, but should not influence the price too much.
As a reference, Dmitry Markov (unrelated to me NYC based auctioneer and dealer of Russian awards) had a complete boxed 1st class set of this order for non-Christians by (a more sought after maker) KIEBEL at his 2012 auction with a $50 000 estimate.
As of the pricing of your item - it is hard to say with any degree of certainty, because of the current state of Russian antique market, but probably $5-7 thousands at full retail would not be a gross exaggeration.
I hope this helps.
