I pulled a 1/2 worn Maxxis back from the scrap tyre bin at a tyre place last year, it cost me a £5 donation to their tea club but that tyre was destined for recycling so it should have been free really. ...and by 1/2 worn I mean half the total tread height had gone (not half the use-able tread).
I've since found that an almost worn out Maxxis IT works pretty well on both mud & stone, well... good enough for green laning. I'm still running it now.