I just had this question myself, and found this unanswered thread, so. Here’s my thoughts:

Looking at Aryxymaraki’s excellent Mining Rules document in the downloads section, we find that a team of miners can clear a 10x10x2 (10 high, 10 wide, 2 deep) in a week, for 40 GP.

Sounds good - he arrived at that by assuming half the cost of building a stone wall was getting stone out of the ground, then adding some cost for bracing.

In fact, we can doublecheck that against D@W:C pg 81 - siege mining. A siege mine that has the chance of collapsing and killing everyone once a week costs about 5 cp per cubic foot excavated; these mines cost 2 sp per cubic foot.

Works for me.

So - excavating 200 cu ft costs 40 GP, we need 800 cu ft more to make our 10x10x10 room, that’s 40 * 5 for a total of 200 GP for a rough space, plus bracing.

That gives us a base cost of 200 GP for a hole in the ground - what’s the other 300GP?

The dungeon entry has a flagstone floor and “rough hewn walls”, which I’m going to take as meaning a rough stone bricked wall, rather than just the wall left where we stopped digging.

Other places on the 'net and at Autarch assume an interior keep wall thickness of about a foot; we’ll take that and run with it.

If we put a foot thick wall all around our room, our room becomes 9x9. We need to excavate a 12x12x10 space, in reality, to end up with a 10x10x10 room.

Our base “hole” cost is then 288 GP of a possible 500 GP.

Stone walls cost .25 GP per cubic foot. We’re making 4 11x10 walls (to meet up in the corners); that’s 110 GP.

We know our flagstone floor costs 40 GP from the book.

We’re at a total of 438 GP of a possible 500 GP.

Pretty darn close. We could drag another 40 GP out of doing the ceiling in flagstone; that’s 478 GP; that leaves us with 22 GP for various stuff; perhaps hiding the buttressing, or doing it in stone, or what-have-you. Frescoes of suffering PCs or empty torch sconces or big devil faces or whatever.

Or we could lose 20GP more by assuming we had to dig out a half-foot deep space for the floor and ceiling each, which gets us to within 2 GP of the 500GP cost.

So, given this, I’d agree that you figure out your cubic footage of dungeon and then pay 5 SP per cubic foot excavated for a dressed dungeon, plus whatever for stairs or special features.

The mage on a budget, or perhaps looking for a more authentic cavern experience, might just pay the 2 SP per cubic foot and save a bunch - I don’t know that the monsters who end up inhabiting the joint will care, but I would expect a ton of bad reviews on Yelp! from looting parties.

*“I could forgive the kobolds on level one, I understand these dungeons are often franchised from the same mage and corporate often dictates the menu, but the walls were practically bare - just one hex over I could go to Barluk the Mad’s and get much more authentic near-death experience.”*

Also, for what it’s worth, I think any mage player would be well served convincing their judge to use those Mining rules - mining out a vein and then reusing the empty space as dungeon is a killer idea.