I found it very useful to not think about percentages of given price, but the actual situation the hagglers are in. Once a peddler-pc wanted to buy some iron and rent a smithy to craft something and wanted to pay with a gold piece (I placed some that were minted in Glethra - as a plot hook). Unfortunately, there wasn’t much the smith could do with a gold coin, so It took our peddler a successful manipulation roll to get the equivalent of 7 silver in rent and iron. Dalb, fully aware that the PCs had no torches and inside weatherstone was dark, asked double the price, if not more.

Remember there is no economy in the Forbidden Lands (yet?), so favours and goods in demand will be common. Also let them know that there is a point where coins become heavy ;)