If you have the 40% surcharge bonus from ascensions, during better deals you get (base value x 240%) x 1.25 or base value x 300% for a surcharge.
But if you look at the value the enchants add with the enchantment establishment talent, they always increase in value by approximately 341.5% when surcharged. That's roughly a 14% increase. 300% increased by 20% would be 360%.
For reference: (platinum glove photos) enchant base value was 15,000,000. (Enchanted value) 141,587,271 - 90,367,126 (unenchanted) = 51,220,145 / 15,000,000 = 3.415. It's consistent (across t14 at least).
I will check the values outside better deals and update the post if they behave abnormally.