I would never buy a used Helmet, but jacket's, pants, gloves etc are fair game. My current jacket, pants and boots were all bought used, My gloves on closeout.
As for prices that's up to you what you want to spend and the condition the items in. I grabbed a older speedmaster jacket in great shape off kijiji in early 09 for $75 and I'm still wearing it, Teknic chicane suit from here for $150ish IIRC and I wear the pants daily (hate the jacket). My Alpinestar smx boots I got in 04 or 05, used off ebay in great shape, since then they've Rezippered twice, re-velcro'd twice and freshly re-heeled this spring and good for at least to the end of this season.