Settlers' production isn't controlled in the XML, hence why (as you've probably noticed) there's no Cost column in their database entry. There are two possible workarounds:
(1) Create a dummy policy with the SettlerProductionModifier column set to your preferred modifier, and use Lua to assign that policy to your civ at the start of the game. (You can see my Tang mod for an example of something similar.)
(2) Prevent your civ from building normal settlers (probably with the PlayerCanTrain GameEvent in Lua), and instead give them a pseudo-standalone unique unit that acts like a settler (e.g., the Found column in Units controls whether the unit is allowed to found cities or not). Pergamon, from More Civs' Greek split, could be a useful reference for creating a standalone unique unit.
