In GlobalDefines there are the following lines mixed in through the file:
Most of those have nothing to do with Settler production. They're the weighting system the game uses to decide which hex to plant a new city on; 15 points per unit of food the hex gains, 8 per point of Happiness (from Natural Wonders), 3 per point of production, +3 if it has a strategic resource, x1.25 if the location is on a coast, and so on. There's also a 12/6/2 weighting based on how far a hex is from the intended settling site. If you turn on "recommend city sites", you'll see the site that had the highest cumulative value this way.
Now, as to what determines the cost of a Settler, it's very simple: it's a fixed amount (in GlobalDefines) plus a fixed amount per point of population in the resulting city (also in GlobalDefines) plus 40% of the building cost of any "free" buildings the city gets (that is, if a Barracks costs 100 and you're starting in an era where all new cities get a Barracks, then Settlers cost 40 points more than normal). This value is then reduced based on a value in the Eras table (I think ConstructPercent, but I'd have to check), just like all other units' costs are reduced for late-era starts.
In my own mod, for instance, I lowered the amount per point of population by 33% and lowered the building multiplier to 25% of the building's cost, because my mods require late-era starts with lots of extra buildings. Note that all of that only applies to games starting later than the Ancient Era, where cities start with free buildings and extra population. I'd give you the variable names, but I'm at a business conference and don't have my mods in front of me.
If you set a Settler's cost to a non-zero value, it'll use that cost no matter what population or buildings the new city would start with; it'd be very unbalanced if you were to do something like this in a later era, since chances are the cost would be less than the Settler's accumulated value.
