Best thing to do is call the saddler and ask how much they would give you then add £150 thats what I did! My Jaguar was £1600 ish new and I sold it in excellent condition for £850. I guess the fact that the Momentum is a new design will work in your favour as there is'nt many about second hand.
I may be wrong what do I know!?! I think its very dependant on how many people are about at the time that want that kind of saddle. I sold my GFS on Ebay which I brought for £500 for £350 two years later and I thought that was quite good but then it was a cheaper saddle.
Not sure! I really like it to sit on, is sooo comfy, and fab to jump in, but I think I need something slightly straighter cut as have short legs and it encourages my whole leg forward.... Also, not sure G likes the Cair.... I do love it, just not sure at the mo... may well keep it, but am just interested on value if I decide not to!!