Is it obligatory to fulfill a vow to pay $250 each time one enters pornographic websites, or is expiation for a broken oath sufficient?

Your entering these websites is a reprehensible act from which you must repent. Bear in mind Allah's observation of you, His encompassing presence around you, and His knowledge of your secret and manifest affairs. As for the vow you made, it is a vow of obstinacy, and if you break it, you have the choice between fulfilling what you vowed or offering an expiation for an oath. You are not obligated to do more than an expiation, as long as your wording does not imply repetition.