Divz, correct on the account that you only buy the equivalent SSF that will equal your total exposure if you bought the actual shares.
So in ur example if you were going to buy R18,000 worth of underlying shares, you can do the exact same transaction with SSF and only put down the margin of R1,800 (if only 10% is required). The catch comes in with the fact that people generally will buy R18,000 worth of SSF cause they have the funds and that is equivalent to R180,000 of the underlying. Then a 10% move on the underlying will wipe your total margin and your position will be closed. If u only bought R1,800 worth of SSF and the share goes down to Zero your R18,000 will also only be wiped when the share reaches zero
The main diferentiator in above example will be what you do with the balance of your cash! All things being equal you will loose out if you left it in the money market at 8% whilst paying around PRIME on the SSF borrowed portion