Delta's rule on SDC is $50 - any fare (even award tickets), any person ($25 if ticketed before 01Aug). This fee is waived for Golds and Platinums. Can only be done 3 hours before departure. Only Golds and Plats can do same-day standby (and then, only when the flight is sold out, as they'd just do the free SDC for them). Sounds very similar to CO's model, just the price difference (and that CO can standby).