Your best bet is probably to contact the Greystone directly, as pricing may change from year to year. It also may depend on the day of the week, the time of the year (peak wedding season or off wedding season), the number of rooms you are using, and whether or not they have any promotional offers you can take advantage of.
It is a beautiful space, and they do a terrific job. Annette (the proprietor) is very organized and helpful. Many couples are also taking advantage of their new chapel and having their ceremonies right there on location.
I do know the Greystone is popular enough that many couples book their weddings 2 years in advance just to ensure their date. So don't hesitate to conatct Annette; she'll be more than happy to answer any questions you may have.
Wishing you and your fiance all the best as you make plans for this very special celebration.
Cost varies from place to place - some expensive and some cheap - like DJs or photographers vary in price; Generally you get what you pay for; In choosing a venue ALWAYS do research on that venue like googling for reviews and ask the venue for 6 past clients to contact and ask about their day; A place may look nice like a picture of some flowers but looking nice does not always mean it works well or the behind the scenes or food is good.....always look beyond what apparently looks nice(no reference to Greystone Castle by this comment by the way)