You want to give enough BP that kingdom building isn't too slow.
The Kingdom Building rules point out:
It's not easy to start a kingdom—probably the reason everyone doesn't have one. If you are founding a kingdom on your own, without an external sponsor or a fantastic windfall of resources, the initial financial costs can be crippling to PCs. Even building a new town with just a House and an Inn costs 13 BP—worth over 50,000 gp in terms of stone, timber, labor, food, and so on.
I don't know if an Inn is the most cost-effective building to start with, but it seems like a reasonable choice (as well as being a logical one: Many towns start around a lone inn that accumulates more services). Lets consider what a base kingdom that started with 15 BP would look like after a month.
Since Economy is the most important stat for growing a kingdom, I am going to assume four PCs who each take a role which they have a +3 to the appropriate stat to add their bonus to Economy, and the rest of the roles are filled by NPCs with a +1 to their relevant stat.
- Ruler: +3 Economy
- Councilor: +1 Loyalty
- General: + 1 Stability
- Grand Diplomat: +1 Stability
- High Priest: +1 Stability
- Magister: +3 Economy
- Marshal: +3 Economy
- Spymaster: +1 Economy
- Treasurer: +3 Economy
- Warden: +1 Loyalty
Totals: +13 Economy, +2 Loyalty, +3 Stability
With the Inn and House, that becomes +14/+3/+3. Being goblins, the alignment will probably be CE, which grants +2 Loyalty and +2 Economy. That brings us to a final score after the first (partial) turn of +16/+4/+3, with 2 leftover BP, and a Kingdom DC of 22 (the minimum for a kingdom).
The first full turn
d20+3 vs DC 22 is only a 10% chance of succeeding, and a 65% chance of failing by 5 or more. Unrest will increase by an average of 1.725 (Rounded to 2, imposing a -2 penalty on all future checks).
- Consumption is 2, which eats up our two leftover BP.
- We have no magic item slots.
- We have no negative kingdom stats.
We can't afford to do anything in this phase except change edicts. I'm leaving all Edicts at "None", because I can't afford otherwise (except Taxation, where my Loyalty is already really low).
I'm not depositing or withdrawing anything, or selling anything, since all that could have been done before founding the kingdom. All that's left is to collect taxes.
The tax check adds
(d20+16-2)/3 BP to the treasury. That could be anywhere from 5 to 11 BP, but on average it will be 7.85.
This phase is impossible to model, and is also the best way to meddle within the rules. Rather than rolling for an event, just pick a good one.
The second turn
I'm not going to break it down as much again, but you're starting this turn with two more Unrest and ~6 more BP than the last turn. That makes you more likely to increase Unrest again, and doesn't give you much BP to actually do anything with, although building a new House will get rid of a bit of that Unrest (as will finding a NPC to be the Royal Enforcer)
Obviously, this example may bear no relation to your PC's actual kingdom. They also may make other choices, such as boosting Stability-granting roles at the expense of Economy. That may cut off the Unrest death-spiral, but it will slow their BP growth even more. In other words there will be almost nothing to do to grow the kingdom for a long time.
This may or may not be an issue. You can use this as incentive for the PCs to go questing for more gold and items (or other benefits, such as a friendly nearby fey that grants the Kingdom +1 Stability by watching the borders) to boost their kingdom. But if you do too much of that, then the difference between the rounds/days of adventuring and the months of kingdom time means you're almost never dealing with the kingdom rules. Conversely, if you focus too much on the kingdom, a lot of in-game time will pass with very little happening.
The rules do suggest:
If you're running a small, self-starting kingdom, the GM may allow you to turn your gold into BP at a better rate. You may only take advantage of this if you don't have a sponsor; it represents your people seeing the hard work you're directly putting in and being inspired to do the same to get the kingdom off the ground.
This rate starts at 1000gp worth of stuff per BP (which is much better than the default 4000gp), but unless the PCs are high enough level to have several thousand gp worth of items, that still won't be very helpful.
Starting a kingdom from scratch is hard. It will take a really long time to build up to the point that you can afford to start claiming any nearby hexes, let alone building any interesting buildings. Until the PCs have enough personal wealth (or other resources) that they will have 10+ BP leftover after their first turn, it's probably not worth starting to use the Kingdom rules.