Well its a 3% chance to gain 100 mana over 10 seconds. Lets calculate this effect in a chain casting situation.
Its equal to a 1% chance to get 300 mana. So say it procs once every 100 times
2 sec cast time: 300 mana per 200 seconds: 7.5 mp5: 37.5 mp5 for group
1.5 sec cast time: 300 mana per 150 seconds: 10 mp5: 50 mp5 for group.
Again, this would be best case scenario, as if you proc it twice in a row, it probably does not stack and you probably won't be casting 100% of the time, or cancel casts.
Still its quite some mana regen if the whole group can benefit from it.
Some information on if and how the buff stacks would be nice. 4 possibilities:
1) while buff is active, it can not proc again
2) while buff is active and it procs, it will only reset the buff duration back to 10 seconds
3) while buff is active and it procs, it will add 10 seconds to the current bufftime remaining
4) the buff stacks
Also what if multiple partymembers got this enchant? Possibility 3 and 4 would make it 100% as effective as the calculation, while 1 and 2 would reduce it with possiblity 1 being the worst case scenario.
So lets calculate how bad the buff can be in the worst case scenario:
10 second buff, assuming a 2 sec casting time, 4 casts finish while the buff is active (5th would finish after the buff has expired). It procs 3 times every 100 casts, 4 out of 100 casts are wasted while the buff is active. Assume its a 100% chance to regain 3 mana per 10 seconds, 12 times it will be wasted during the 100 casts and 3 procs:
36 mana lost over 100 casts of the 300: 12% effectiveness lost
Reducing it to:
2 sec cast time: 6.6 mp5: 33 mp5 for group
Did not calculate how much it would affect a 1.5 sec cast time.
Still pretty good.