In his lifetime Slania must have signed many thousands of stamps and covers, as signed items are common. The premium charged seems to be what the vendor thinks he can get away with. Sometimes hardly anything extra, while at other times it will be a stupidly high premium.
Currently on eBay
signed Slania items vary from a low of $8 for an unused Swedish postcard to a high of $700 for a sunken die proof...and everything in-between. Of course, the item that has been signed will greatly influence the price as well. GLENN