Exactly. Cups are a measure of volume, not weight. A cup of water (or other liquid, like milk) weighs about 250g. But a cup of Splenda weighs much less than that because Splenda is not as heavy as water.
One way to do it is to search in Google "1 cup Splenda in g". If Google itself doesn't calculate it, it will usually turn up a few websites and forums where other people have worked it out. Look at 3 or 4 of them to get a good idea of an average answer (if you just look at one, that person might have gotten it wrong!). That's how I do it when I convert recipes for here (I cook in cups normally).