I just ran Comrades 2012 and the rough costs that were related directly to Comrades were:
Race Entry US$180/£120ish plus you need to buy a timing chip as well.
Flights
London-Johannesburg (return) - around £550 per adult Air France (via Paris). Direct London to Joburg flights were around 700-800
Joburg-Durban (one-way) - around £40 per adult on a low-cost airline (Kulula)
Hotel in Durban was around £80 per night for a 2 bedroom apartment with breakfast overlooking the ocean
Food costs in Durban were fairly modest for excellent food - I'd be surprised if we even spent £20 per person per day for food and drinks
Transfer from Durban airport to Durban was about £8 per adult by mini-bus each way
Bus travel to the start was around £5
I also indulged in Comrades branded clothing which was fairly pricy at, e.g., around £25 for a technical top
All other things in Durban were very reasonably priced, e.g. we went to the uShaka aquarium/dolphins/seals and spent around 4 hours there for £20.
The only thing I found expensive were taxis
I ran it as part of a bigger holiday in SA and spent 4 nights in Durban (arrive Thursday. leave Monday)
You could drive from Joburg, but it would take 12 plus hours and car hire would be £40 plus fuel - wouldn't really recomend it as you would already have been travelling for 15plus hours by the time you get to Joburg
Enjoy it!