There isn't a real good answer to adjusting the cost of veteran leaders.
There is a cost that can be modded for rookie leaders. this is located in there stat file.
Using the Forest Liege for example, you would open Disciples III\Resources\Profiles\stats\elves_leader-forestliege_stats.char using notepad.
Scroll down or search for the cost entry which is 500 by default.
Looks like this: cost 500;
Just change the 500 to something lower. I have not seen a way to adjust veteran leader cost yet and I have done a lot of modding. I'm not positive if the cost of a veteran is a straight multiplier of the cost of a base leader. Often veterans cost about 2000 in Hard difficulty. If it was a straight multiplier, making the cost of a rookie leader 200 would make the veteran cost 800 in this example. Only I don't think it works like that. I haven't verified this, but It might be that the veteran leader cost is independent of the base leader cost in the stat file.
For one thing. The armor and amulets, rings etc.. that a veteran leader is wearing increases the veteran cost as well. So lowering the cost of armor would lower the veteran cost to some extent. But that screws up balance elsewhere.
Like I said, no good answer, but I thought I'd say what I know.
The import/export leader isn't available anywhere as its probably not coded anywhere in D3.
There is a way to make veteran leaders affordable, but its not as simply as lowering cost. One way is to stay in the previous act long enough to buy armor, rings and amulets. Armor and amulets, etc are brought to the next act with the main hero, unlike potions which are lost. They only sell for half price of course. But if you bring 6000 worth you get 3000 when you sell at the start of the next act. Enough for 1 veteran anyway.
One thing I mod in my game, is I find a building that isn't used for any function near the start of act 2. Then I mod that building to give 2000 gold. It could give any amount really. The problem with that solution is that building type may appear in other acts in different locations. It can't be isolated to just one act unless the maps is custom. Modding a building like this has a few more steps than adjusting a start up leader cost.
These are some work arounds to get gold for veterans. Not a perfect solution, but works in a pinch.