Given a flight route and date/time, how can I find out the price of NRSA (non-revenue space-available) tickets for Delta Air Lines?
You ask the source of the non-revenue ticket.
It is likely there may be no one set rule on what these might cost - I have talked with employees of an airline in the past (not Delta) and they had different options based on their position and length of service with the airline. One had immediate family could fly free on standby, another (same airline) had immediate family could fly for 10% of some cost. Others (with different airlines) had similar arrangements.
Your best (possibly only) source of this information is whoever is arranging the ticket for you.