My mom tried desperately to get me to use her (my) old crib mattress--- It has been off-gassing for 32 years! HA! I just had to say no to that one. Looks perfect, but the little voice in my head screamed UM NOT GOING TO HAPPEN!!! so I bought a brand new $300 mattress from Naturpedic.. (It's being delivered today - Eeeek) I went off of a friend's recommendation, and I probably spent WAYYYY too much, but I didn't have to buy a crib, my dad fixed up the one that my mom, aunt, myself, and my sister slept in... Now the sides don't drop anymore and it's all ready for it's fifth baby (my first!)
I was curious about your question though, and did a (very) quick search, and found very vague info on how certain chemicals never disappear from mattresses. I also know that everything off gasses, so if you have a new carpet in the room, or fresh paint (that wasn't low voc) or whatever, that will off gas... and what about the bumper guards and sheets and stuff! (I went for organic bedding too)
BUT I totally have a pack-n-play brand new - and I'm sure my baby will be sleeping in that all the time so maybe I'm a hypocrite? Or the brand new car seat/carrier, or baby swing... or the fancy new stroller... Uh oh, now that I am thinking about this I just may open all my boxes while the weather is nice and air the all out in the sun!!! (i'm totally not being sarcastic - a mom can get crazy over these things!!!)
I think that a $100 organic mattress sounds like a great deal, and the $30 used one is a respectable option! Either way make sure there is a fan in the baby's room to keep the air moving, i'm sure you'll be fine!
I don't think my rambling at 6:30 in the morning before my coffee has helped you at all, but I am interested in the subject, and I think that both of your options sound great!
Best of luck!