The other half is a massive fan of these (he had two today)! However, unlike the other Christmas drinks (Toffee Nut and Gingerbread), they don't just use a couple of pumps of the flavouring; they use a creamy pre-mix for the eggnog which is really quite calorific! I can't be sure how much fresh milk they add - perhaps ask your barista - but you'd be safer just to syn the whole thing and use your healthy extra A elsewhere. Nutritional info for all Starbucks drinks can be found on their website, and as others have said, 20 calories = 1 syn.
As I posted elsewhere, the Eggnog latte isn't on the nutritional information sheet, I assume that is because it is a seasonal drink and only available for a short few weeks (or sometimes days depending on how long the stores supply lasts)
The Eggnog latte's aren't made with milk, they are just made with the premix, I have no idea what they are syns wise, but they contributed to my 1.5lb gain this week!
For the toffee nut and gingerbread latte I would take the highest syrup syn value possible.