Scaling can of course be either Linear or Exponential
Exponential works well to make early growth easier "low cost for first few then large increases" eg - (Citynum-4 (min 1)) squared gildar per turn.
Linear works well for longer games, as it makes growth early-on hard, but late-on easy (as you have more gold, etc).
Then for possible technologies:
1) Provinces - the skill of managing provinces, parts of your kingdom that are semi-independant, is a task the romans learnt well. It drives down administration costs while keeping them "yours" - (1/2 cost maintainence - )
2) Colonies - granting independence - one of your cities becomes a 'neutral' city... but begins with a very friendly attitude to you and a full trade agreement of their resources - (no maintainance from that city)
3) Advanced administration - learn how to better manage local cities from your base city - (discount all cities up to 9 squares away from first city)
4) Super-duper advanced administation - up to 14 squares away
5) Efficient administation (base calculation ignores more cities - larger effect on exponential measurement).
(3/4 based on distance, 5 on number of cities - do you have a tight ring or a spread-out blob of cities?)