I'm also only a user of this forum.
I could not find any information regarding your medallion. I only found a 22 carat gold medallion that was sold on a platform similiar to ebay in South Africa during 2011.
A search on http://www.southafricanmedals.com/ also ended up with no result.
According to your pictures i would guess that your medal contain 1 ounce (.925) silver.
The minimum value will therefor be the current spot price for silver.
Cape Mint (Pagliari) used (.925) silver for all other medals, as well as sets of medals.
I do have a few of medallions of the South African Administrators.
Your medallion might be included in 2008 Herns Catalogue - Medallions of the Zuid Afrikaansche Republiek And The Anglo Boer War - Soft Cover (South Africans) The catalogue, however, will cost between ZAR400.00 to ZAR 500.00