Since I do them with Fireteam medallion, I'd say at least 1 out of 10 PEs gives me something purple. If not from the chest (that's rare, if ever), then from boss or retinue. Most of the time it's just some Y1 stuff without perks. Could be masterworked, though. Also they can drop prime engram, but given how RNGod treats me, it probably will be same slot item with increment of 1, or it will have ridiculous perks. Or both. Currently I'm on five in row titan marks. Honestly, I'm doing PEs as source of radiant seeds for weapon frames, some of them are very quick and easy with Riskrunner (anything that involves Fallen). But that's it. Some could be daisy-chained, in case boss&retinue haven't dropped you more than 10 seeds. I prefer Winding Cove&Firebase Charon, as they occur one after another, the latter has either Either or Glimmer, so it's fairly easy.