The TurYol bosphorous "cruise" is 12 TL, and leaves every half hour. Be there a half hour…read morebeforehand for boarding, and if you can, snag a seat on the port (left side) of the boat for the best views. The announcer only discusses the sights you're passing in Turkish, and the PA is very difficult to decipher so if you're not Turkish-fluent, don't expect a lot of guidance. Tour guides can get on for free, though, so if you have a guide with you you're golden!
Great opportunity for some nice photos of all the riverside palaces in Istanbul, and to get a sense of just how permeated this landscape is with history. You will be packed in, though, with tons and tons of families and plenty of rowdy children... So if crowds aren't your thing, spend the extra cash and go on a more private Bosphorous excursion.