this chart is handy. in the interests of full disclosure, i copied it from another cruise message board site.
Platinum Balcony Discount When Booking More than 6 Months from Sailing Date:
$ 50 * for sailings of 3-5 nights
$100* for sailings of 6-9 nights
$200* for sailings of 10-14 nights
$200* for sailings of 15+ nights
Emerald Balcony Discount When Booking More than 6 Months from Sailing Date:
$ 75 * for sailings of 3-5 nights
$125* for sailings of 6-9 nights
$225* for sailings of 10-14 nights
$225* for sailings of 15+ nights
Diamond Balcony Discount When Booking More than 6 Months from Sailing Date:
$125* for sailings of 3-5 nights
$225* for sailings of 6-9 nights
$325* for sailings of 10-14 nights
$325* for sailings of 15+ nights
Diamond Plus & Pinnacle Club Balcony Discount When Booking More than 6 Months from Sailing Date:
$150* for sailings of 3-5 nights
$250* for sailings of 6-9 nights
$350* for sailings of 10-14 nights
$350* for sailings of 15+ nights
note that if you book within 6 months of your sail date, or make changes to your reservation within that time (other than changing rooms in the same category), they will subtract $25 from the discount.
these days many promotions are not combinable with the C&A discount. on-board booking credit is, but many other "sales" and "promotions" are not, and as far as i can tell, anything that gives you on-board credit (except for the on-board booking) cannot be combined with the C&A discount. you'll have to read the fine print with each sale/promotion/offer.
sometimes the website will give you the best rate, sometimes not. for example, a couple of weeks ago i booked a cruise on grandeur of the seas, which offered free gratuities ($136, in the form of OBC) or my discount ($125, for a diamond for a 5-night cruise). i could not get the website to give me the OBC; it would just default to the diamond discount over and over, even when i checked the box for the OBC instead. i didn't bother to call and get it fixed for the $11 difference, but if the difference is significant, you might have to call to get the lower rate applied. eskimo is right that if you're platinum, the current promotions are often better than the offered discount.