OP, are you sure that the TOTAL price for 2 tix wasn't 500k, rather than it being 500k each. ATL-JNB is a direct flight so even DL's terrible award engine isn't likely to screw up the pricing.
If you are truly willing to pay $4k per ticket, the. Redeeming for medium level for 250k miles per ticket is worthwhile, IMO. In this case you are getting a value of $0.016 per mile which is a fair value for DL miles.
If you don't have the time or know how to search the various partner airlines and routings, you might benefit from using one of the many reputable award booking services to get you there for 140k miles each.