www.usps.com had a calculator which will give you the cost.

I assume that you cannot get US stamps in South Africa, because it is illegal to sell them anywhere outside US.
So, because you cannot get them, you should include a couple of IRCs, which stand for International Reply Coupons. It is the same as stamps. All you have to do is just including them with your letter in the envelope.

I reside in South Africa, can anyone inform as as to whether its possible for me to include US forever stamps on my SAE when sending to a US celeb?
If so, can anyone perhaps shed some light as to how many I should include?
I would more than likely include 2 8X10 photographs, so any info would be greatly appreciated!