I suspect that the reason for that oversight is that "everybody knew" that sap recharge crystals had a cost in dappers. Therefore, when the change was made three years ago, no one rewrote the description of the spell.
Actually they cost dappers at least since release... the only change made 3 years ago was that they no longer create crystals but charge the item directly.
But I second the notion that the dapper costs should be noted on the spell. (it's one of the few minor dapper dumps after all)