I would suggest you use something like this @ +-R1 000:
http://www.samsontech.com/products/productpage.cfm?prodID=1810
If that is a bit to pricey, you can also try these "cheap" mics you get at PnP for instance, it will still be better. None of the cam microphones will be good enough and there is only a few headset mics that is adequate (depending on how professional you want it to sound) My Sennheiser USB headset has adequate quality, but for less money you can buy a better mic like the samson.
The ultimate for a budget is the Shure SM58 but that is @ R1 500
You should however be able to get some decent quality mic for around R500 even go for second hand