It really depends on which model of rook rifle this one is and the level of finish and condition. The basic rook rifles are mostly center hammer, side lever, break actions. A very highly ornamented one (full engraving coverage, nicely figured wood) in excellent condition from a top name maker will bring almost that much. There are hammerless, top lever, break actions, which to my eyes, are the best looking of all the variations. They are not nearly as common and tend to bring better prices for a particular finish level and maker's name than the first version. There are also side hammer, top lever, break actions, also quite handsome and they also usually bring a better price than the center hammer versions. Finally, there are the miniature Martinis and miniature Sniders, neither of which are break actions.
By way of comparison, Vintage Doubles has a Holland & Holland center hammer rook rifle listed on Gunsinternational.com for $2800. It's been there a while and I suspect he'd take a good deal less, but it will give you a base line to compare to the one you've been offered.
One additional suggestion - be careful about getting into these. They're like potato chips. It's highly unlikely you'll be able to stop at one.
