Slitherine

Can anybody tell me how the amount of silver/ gold required is calculated for Vassalisation diplomatic option is calculated?
Why do some states require just silver and others Gold and Silver? eg for the Odryssian Thracians to make a vassal of Persia 500 million silver is required (a trifle ambitious) but for the Abderites only 96 gold + 400k of silver.

Can the calculation be modified?

It's based on size, relationship and experience of diplomat - possibly other factors too but these are the main ones I think.