10% for most gold sites.
My own site I auto put 10% of all sales for disputes/chargebacks and consider the money lost.
So if say we process $3k a day, I automatically consider $300 of that to be lost.
Starting out I'd guess u'd be lucky to process $200-$500 a day, as a new site you will often be targeted by fraudsters & make mistakes which typically leads to a anti fraud rate of 15-20% which is why not many gold sites last long.
Standard gold margin is between 18-28% on both rs3/rs07, when ur fraud alone is eliminating ur profit margins, ur business model tends to collapse.
Buy & selling runescape gold is a risky game to play starting out.