I don't know specifically about Afghanistan, but I just purchased a SIM from Ekit, for upcoming trips to Greece and Costa Rica. Costs are reasonable. I bought the "SIMple Card" for $9 + $3 shipping, and it includes a $10 credit balance for calls. Afghanistan is included in their list of countries.
If you get one, get it through their Ebay site, not their primary website.
A great forum for this kind of information is
www.prepaidgsm.net.
Good luck!