It will be some time before Dominion is active, but enough people have asked about creating domains or promoting domains that I think I can give some rough guidelines before automated systems come in. Currently, the rank of domains, such as a barony versus a duchy, is determined by its land statistic for size and the rating of each of its building type, which along with population, control the income. The rough categories are as follows, from Barony to Arx, in social rank:
6: Land 10, Buildings 75, Population Units 400.
5: Land 100, Buildings 250, Population Units 2000.
4: Land 400, Buildings 500, Population Units 4000.
3: Land 1000, Buildings 1000, Population Units 8000.
2: Land 2000, Buildings 2000, Population Units 16000.
1: Land 5000, Buildings 5000, Population Units 40000.
These numbers won't mean much to people right now. There's 5 different building types currently: mills, lumber yards, mines, housing, and farms. Each of which has a rating, which determines the relative productivity- it's abstract, so it could mean a few exceptionally well developed buildings of that type, many okay ones, or so on. But the total overall cumulative value of the different buildings is what we're looking at for the rating. So when a barony has enough buildings that add up to a rating of 250, it could start to rival a county, if its population has increased to at or over the 2k mark as well, and has expanded to 100 or higher land.
Currently, this is only done in GMing, largely through actions. A rough ballpark is 50 economic resources per rating of a structure increased in dominion. So the cost of a county going to a march would be 250 rating in buildings, or approximately 12500 in economic resources. That's a bit higher than an equivalent of a 5 year GDP of the target economy. This can be done incrementally, and every action will increase the ratings of domains as I GM it by hand, eventually it'll be automated and use a formula like:
Roll to increase a building in dominion, costs 10 AP and 100 economic resources per attempt. Command + stewardship at 30+(current rating / (1+(social_clout * prestige mod))), increase building power by 1 for every 10 over target, round down.
But in the meantime, this will happen purely in actions, and the costs will be greatly mitigated by investment of AP and communal pooling of skills.