
Mathius |
According to ultimate campaign a house will cost 1290 GP and that includes all furnishings. The example house has a max size of 40 squares so that works with 30 x 30 house.
It is not stone house nor is is particularly upscale. As DM I would feel justified in doubling the price for stone house and I would also double the price to put something underground. That means 1580 for house dug into a dirt or clay ground while digging into stone would be 3160. I would not allow underground construction in a swamp.
I would also charge more if it not in or near a settlement of at least 60. Hamlets tend be one or two clustered families or a small projected dedicated to one thing like logging camp and are not good places to just plop a new thing. I might double price again if you more then a mile from settlement and double it again for each doubling of distance.
That said it would take a lot to get up to 20K.
As a plot point he might be trying to rip you off and you could make sense motive check or appraise check to figure that out.
The DM may also be trying to balance you WBL and needs a reason to take extra GP from you. I would talk OOC about that one.
The DM may not know what is in UC.
The spot you have chosen may be valuable for some other reason such as a spectacular view or defensibility and you paying premium for it.
The NPC might build your house extra fast be charging for that.

MagiMaster |

If you go by the Stronghold Builder's Guidebook (which I know some people have problems with), a luxury bedroom suite, which includes pretty much everything except the kitchen sink (and the rest of the kitchen), would cover approximately 800 square feet (so close enough) and cost 25,000 gp. The stone walls are free underground. There may or may not be a surcharge depending on the details of where underground.
On the other hand, a basic bedroom suite plus a basic kitchen, which is more self-sufficient and somewhat like a farmhouse, would be 2,800 gp and also take up about 800 square feet.
So, it'd really depend on how ornate you want, IMO.

Orfamay Quest |

According to ultimate campaign a house will cost 1290 GP and that includes all furnishings. The example house has a max size of 40 squares so that works with 30 x 30 house.
It is not stone house nor is is particularly upscale. As DM I would feel justified in doubling the price for stone house and I would also double the price to put something underground.
If I remember correctly from my long ago Intro Engineering classes, underground construction is more like 10x more expensive than 2x.
That would bring it up to the 20K range very quickly. Basically, excavation is expensive, even with modern technology. If we're talking about peasants and shovels, it's worse.