Hmm gram. 700-800 for flights etc (depends what you book and when), accommodation, you could find a dorm room bed in a nice hostel for 15-20 quid per night, a room to yourself 30-something (for both add 1-10 quid per day for buses or taxis), a hotel near the Expo perhaps double that again. So accommodation 150-300 maybe? Food more or less same as it is here.
So all in all around the thousand quid mark ... ish...
Gibelas is a nice hostel where a few of us have stayed but it fills up so book early. Sign up for all the airlines' mailing lists and start watching for flight sales. There are cheap airlines that fly from JNB to DUR but again I guess early booking helps.
Race entry is $180 iirc. Unless you have contacts in SA and can join a SA running club and get a local address to apply from.