The formula for Mana Surge is as follows: SURGE CHANCE = (MASTERY*0.73) / (MASTERY+62)
This is called a "diminishing returns" formula... it means that as you get more and more Mastery, the INCREASE in Surge chance goes down. For example, 25 Mastery points give about a 20% Surge chance, but if we double that to 50 Mastery points, we now only see a Surge chance of a about 32%. The Gems of War system really starts to slow down around 60% as you can see in the graph.
Formulae like this are used in games to avoid imbalances for players at the end-game. An example of a game that uses a similar formula is League of Legends, with its damage reductions from Armor & Magic Resistance.