Being in SCED just means that the resource bid into the real time energy market (which clears every 5 mins), it does not necessarily mean that the resource is cheap to dispatch. The confusion here might be caused by the differences of the ancillary market (PRC) and the energy market (SCED).

Your second paragraph may be answered by this: https://www.ercot.com/gridmktinfo/dashboards/gridconditions. PRC units are available in real time, immediately on request.