If you want to SDC, you would need availability in the same award bucket as your original flight... so for example if you buy on low/N then you can only SDC to a flight with low/N availability. If you have a mid level ND ticket, then you can get on a flight with ND or lower available.

50K is expensive for a domestic award, however you are getting 1.6c/mi which isn't horrible depending on how you usually fly.