usually there's alittle catch either beside or underneath the unit that will allow you to undock it, however, barring that, it will require removal of at least the keyboard and palm rest. there might be more specific instructions based on the model, but this should give you an idea.
http://www.bixnet.com/cohpnoopdrin.html price, plain old CD and/or CDRW is probably between $60-$120 DVD and CDRW/DVD is probably no more than $160 and DVD writer is probably around $200. Those prices are probably a bit high, but it depends on availabilty.
Hope that helps. :D