(ninja'd; this is a response to Yanlin)
I disagree. While "boarding pods" indeed make little sense (actually I can't remember a game that uses them), a shuttle or a ship attaching itself to the target ship and "injecting" a specialized team of heavy soldiers into it makes sense. As far as having battleships in space makes sense, anyways. A team specifically trained to overtake ships will have: a) heavy shields to protect against turrets, b) heavy weaponry to blast holes through the internal walls, c) a crack team of electricians and hackers that can disable the ship's security and open all doors.
For approaching the ship to be captured, several ways exist. Some are even used in games - like disabling the ship prior to approaching (I distinctly remember battleships blowing pesky raiders off their hull in one of the homeworld games) or taking out the turrets that can arc in that direction. You can also employ some Stealth In Space and sneak up on the target. And don't forget settings that have teleporters.
As for cost... let's see. A normal ship would have massive armor that protects it equally from all directions, it would be bristling with weapons, it would have specialized internal defence teams and automated defences against takeover, and quite possibly a lot more stuff. A ship designed to board other ships will be small, it will have most of its armor on the front and "bottom" (unless it's a ramming boarder, in which case only the front), it'll likely have no weapons, and it won't have any additional defence mechanisms, because it's quite literally PACKED with heavy troopers trained for combat inside ship corridors. Therefore, it'll be a lot cheaper than your average combat ship, and costs to refit it between boarding operations would be fairly low.