You will need to change the priority the AI has for training pioneers to cope with the change. The AI doesn't look at the cost and think it is too expensive. It looks a the AI Priority and uses that to make a queue. At least that is how it seems to work. 

I'm interested in how this works out.   Will this give the pariden spell too much power when they can "spam" outposts with magic?

How did you end up modding this?  did you give the pioneer design some equipment that costs 50 gold?  Or some other way?

I think they could get rid of a lot of the city spam by:

1:  Make all the "whole empire" bonuses "one per empire".

2:  Make more "whole empire" bonuses, especially for level 4 & 5 cities, so there would still be as much overall "per empire" bonuses, and there would be more motivation to build big cities.  Or else increase the bonuses from the existing bonuses

3:  Eliminate the 3-turn minimum build time -- again, makes big cities desirable instead of nerfing them and favoring lots of small cities.  This has been requested elsewhere.

4:  Maybe also partly undo the recent change that makes outpost improvements take time in the city building queue?   Perhaps make them take half time and justify it by proclaiming that half the work is being done by the original pioneers who built the outpost and the other half done by laborers from the city.

5:  Tweak the AI to recognize the advantages of fewer bigger cities.

Krax would be OP- he could spam 4 pioneers early and get 5 cities while everyone else is on 2.

The only issue I can see with this are those sovereigns that start with 1k extra gold.  They would have a huge advantage as claiming land/resources early is the most beneficial way to play. 

What I would like to see is your first city reaching a requirement (population limit?) of some sorts to create your first pioneer.  Then another requirement for your next and so on.  This would make you develop your capital first before setting out and finding new lands while simultaneously giving you the opportunity to clear your surrounding lands of the lower level monsters that would go after your pioneers.  

Solymer, I modded it so that factions can't start with 1000 extra gold, they only start with 150 extra gold. 

Kongdej, I am trying to find a solution that we can do until they fix it on their own somehow. In my opinion, this is a simple solution and I think will work. I might even increase the amount to 300 gold for a pioneer, at least for myself.

Can you make it so cities is an upgrade from outposts?

Would work somewhat as I imagined could help the problem, if the upgrade to the outposts costed 3-6 turns of extra production and meaby 20-50 gildar.

Not too happy about nerfing the "Wealthy Ability" myself, its pretty... Meh. As is already, gives a nice starting game, but if its nerfed its a waste of space

Sincerely
~ Kongdej

A small basic fix I think would be to increase the min build time to so 5 or 7. Piooners aren't like other units they don;t get worse over time.

IIRC, the magic outpost spell has a mana upkeep.   They will still have an advantage, but you could say mod that to 2 or 3 mana per turn instead of 1.  That way it's not unlimited.

It's only a flat 50 mana cost, no upkeep required (same as with other outposts).  Pariden outpost spam is already pretty effective once you get a decent mana income, and with the changes suggested by the OP, would make them far and away the most expansionist-happy faction in the game.  If you REALLY want to limit the number of cities in the early game, why not make faction prestige the max number of cities you can found?  Keep the growth limit for prestige so you still grow more slowly if you decide to take all your neighbors' cities, but tying max cities to your prestige, you quite effectively limit city growth in the early game (when your sovereign should be out there gaining levels), and would much more effectively accomplish what prestige was implemented for in the first place.