Unless there're a lot of people travelling, the bus will be much cheaper.
The bus terminal in Playa is a couple of blocks from the ferry pier, direct buses to downtown Cancun run every 20 minutes or even more frequently, cost is MX$45 one way (about USD 4). They may make a couple of quick stops along the way (at the other bus station in Playa del Carmen and in Puerto Morelos), Cancun is the last stop, so you can't miss it. The ride lasts about 1h15, depending on traffic.
Once you arrive, you can either catch a bus (red buses marked "Hotel zone/Zona hotelera" if that's where you're heading) or grab a cab to get to your final destination.
If going to the airport, there are direct buses departing Playa bus station, those run less frequently (every hour or so) so check the schedule ahead of time (
http://www.ado.com.mx). Cost is USD10.
If you're afraid to get lost, Ron Lee's webpage (link in his post) has very detailed useful explanation. No worry though, it's really easy.
Unless you book a van in advance, cab costs will widely depend on your ability to negociate the price (always do so beforehand). I've been quoted anywhere from MEX$400 to twice that amount for an airport ride (took the bus instead).
The bus is comfortable and fun, they often have subtitled movies playing. Word of advice though : bring a warm sweater, jacket or blanket, they overuse the A/C.
