Christi at Blue XT (who's a member of this board) set us up with a Tulum tour that was cheap and very easy. We had a really nice local guy pick us up at the ferry in PDC and shuttle us to Tulum and back. Our guide at Tulum was terrible, but I think who you get down there is just the luck of the draw no matter how you get to Tulum. The price was like $90/person and included roundtrip ferry from Coz, transport, lunch and admission to Tulum and the cave park nearby (aktun chen, I think?). We had absolutely no hassles from anyone trying to sell us anything, which was nice.
Incidentally, I liked the aktun chen tour as much as or more than Tulum. It's about two hours of guided tour through the above water portions of a cenote. The above ground portion of the park is also a nature preserve and they've got a ton of local animals around, which I thought was really neat. If you go, don't try to pet the javalina - they bite!