I think purples are like 15% rate...tops.
There are definitely stats out there ppl have posted. However, as with farming any purples, I literally just try and not pay attention while I hit the sim key, it's just too frustrating. Same with hard node shard farming.
Cantina is the only place where farming is tolerable.
Sometimes when farming blue gear I'll go 0 for 10 or 0 for 15.....
It's about to get way worse with gear 9 and 10