some ms gold subcription bundles or cards come with 2 codes, one for the subscription and another for ms points as low as 300 i've seen to upwards 800. this way you can either use the gold membership to prolong your XBL, give it to someone you know/sell it, etc.
another option would be to call xbox customer support, you might be able to purchase a smaller increment of ms points.
...and of course, if you won't want your little sister to spend $19.99 on 1600 pts you can always get them for her and be her big brother hero.