descr_campaign_db.xml and these lines for merchants:

<merchants>
<base_income_modifier float="1.0"/>
<trade_bonus_offset float="10.0"/>
</merchants>

Just set the numbers higher.

descr_settlement_mechanics.xml for DtC and these lines:

<factor name="SOF_DISTANCE_TO_CAPITAL">
<pip_modifier value="1.0"/>
<castle_modifier value="0.2"/>
<city_modifier value="0.5"/>
<pip_min value="0"/>
<pip_max value="16"/>
</factor>