The thing that needs to be understood about sand as it applies to marine aquaria is that the sand plays a key role in the water quality. Crushed Coral and Aragonite are all mostly calcium carbonate. This material slowly dissolves as it ages in the tank, buffering the water, driving up your akalinity, and keeping pH stable.
Play sand is not always the same type of stuff. Even if it was, you can't be sure of what is in it. It is manufactured for a different purpose, so no controls are in place to ensure the purity of the sand.
Furthermore, the size of the grain is important. Sand that is too fine can trap hydrogen sulfide and dead pockets within the sand bed, waiting to unleash a veritable death bomb when disturbed by a gravel cleaning, etc.
The buffering is the huge key though. You may be saving money up front, but the problems that you'll have with your pH is going to really make you wish you had spent the money. Again, even if you find sand that is calcium carbonate based, you have no way of knowing if there are various nasties left in the sand that could cause all sorts of problems.