How much value do groupsets gain/loose per year?
In my experience, it depends on how much has actually changed. Usually the lower quality groups (Shimana Alivio/Deore, SRAM X5/X7) don't change too much because they are very similar to the OEM parts. However, the more expensive groupsets incorporate the trickle down method you mention. Things like clutch derailleurs are becoming available in the middle price ranges versus the top tier. Adding something like that will greatly decrease the previous year's model, while increasing the new one's.

Most of the time though, it's like a car. As soon as you drive it off the lot, the value drops. Even if it's brand new, as soon as the next year rolls around it's lost value. Marketing and hype are what sell product and new product is always pushed first.