I tested this a bit, not too much though, seems to proc sporadically; Here are the results.
Test 1:
66 Hits; 3 Procs
4.54% Proc Chance
Test 2:
46 Hits; 7 Procs
15.21% Proc Chance
Test 3:
37 Hits; 2 Procs
5.41% Proc Chance
Test 4:
65 Hits; 4 Procs
6.15 Proc Chance
So, combining these results, I've gotten a *[7.47% Proc chance for a total of 214 Hits.]*
Can be a life saver, considering it's level, I'd say it's well worth the 65 gold if you can spare it.