For an all itemlevel 61 geared character, you should have 570 points to divvy up among Accuracy, Alacrity, and Surge. Accuracy is obviously useless for a Sage. If you put 570 into surge, your surge value will be 80.11%. The first couple of blocks of 57 you move from Surge to Alacrity will give you almost 2% cast time reduction at the cost of only about 0.5% surge value. It does get steeper pretty quickly though, to where you're trading ~1.5% surge value for 1.5% cast time reduction when Surge = Alacrity and even worse when your alacrity is above your surge. At 570 Alacrity + 0 Surge, your cast time is reduced by 15.14% and your surge value is 51%.
Yeah, I noticed this when switching my implants/earpiece to the ones I have which carry the Alacrity instead of Surge. Since starting TFB HM there hasn't been an instance where an extra 2% quicker cast would have been "make it or break it" on a clear.
Not that the extra .5% Surge has either. But just in terms of getting the most "bank for my buck" when I am casting, I can see a tangible benefit from the Surge versus the Alacrity when watching my Healing Trance critical 3-4 times over its cast time.