Okay, here goes. I'm paraphrasing most of the text.
Priests need temples, rogues guilds and wizards sources. Building a domain with the 'wrong' holdings will gain you a lot less regency.
Domain Design Points: Bloodline Score + 2d6 for Tainted Bloodlines, BS+2d8 for Minor, BS+2d10 for Major and BS+2d12 for Great. (Please note that this assumes 2nd edition bloodline score; thus double that indicated in the 3rd edition birthright.net pdf.)
Holding and Province Cost: Province 2/level (1/lvl for warriors), Law Holding 1/level, Guild 2/level (1/lvl for thieves), Source 2/level (1/lvl for wizards), Temple 2/level (1/lvl for priests) If a Holding is created in a province not adjacent to a province containing one of the PC's Holdings, the cost rises by 1 design point.
Assigning a terrain type to a province doesn't cost any extra design points, unless the DM rules the change is too drastic.
Domain Asset Cost: Army Units 1 per 8 GB of units, Contacts 1 per 2 Contacts, Fortifications 1 per level of fort or castle, Lieutenants 1 per lieutenant, Loyalty 1 per province raised to high, Trade Routes, 1 per trade route, Treasury 1 per 5 GB in treasury.