The cheapest way to get MKSAP is to buy a copy off someone who is done using it. Since the boards are in August, usually any first year fellows in your new program will be happy to unload it on you, assuming you're happy with a dogeared copy. A slightly older version is not that big of a deal, although I would not go any more than one version out of date (i.e. MKSAP is now #17, so using #16 to study would be less than ideal but probably OK)