Would it be possible to carry moondust, from Sandstorm, in a flask and attempt to use it as a thrown splash weapon, forcing saves against suffocation? If so, how terrible of an idea is this? If building from a budget, what would a reasonable cost be?
The Short Answer
Yes, it's possible to carry Sandstorm's moondust in a flask. Yes, it's possible to use it as a thrown splash weapon. No, it won't induce suffocation; it'll just make your enemies filthy.
The Long Answer
Sandstorm's moondust (23) is a hazard, like green slime (DMG 76) and anchor mists (Du 140). You can no more weaponize moondust than you can other hazards--they're Dungeon Master toys for making the world more dangerous, and, unless character options permit, they're beyond the reach of players.
Further, moondust appears to be something to suffocate in, useful when the DM wants to have nasty trap and water (or green slime or vanilla pudding or hot melted cheese) is inappropriate. The amount of moondust needed to suffocate a creature goes unmentioned, but as the later reference in Sandstorm indicates, "natural sinkholes filled with moondust" (32) are a thing. So while you could bury a foe in moondust and laugh as he died a dirty death, throwing a flask of it on a foe will just ruin his suit and tick him off.
Whatever method you use to attain this item is up to your DM, as is the result when you take the stopper of the flask/break said container. For my game I bent the world a bit and turned the wizard Mordenkainen, from Greyhawk, into a traveling merchant aswel as a mighty wizard.
If anything I would try to treat it as a thrown item resulting in a local Sandstorm/Tornado, which can cause suffocation, slow, blindness ie. at the location it lands. You could even roll a d8 to determine the direction it travels and a d4 for it's speed.
Or if your DM is feeling generous, try attaining it as a magic item that causes a Sandstorm/Tornado under your control?