According to the core rules, a keep costs 50,000gp to build from scratch. There's a piece of art in one of the official adventures that's 50k gp all by itself.
— Matthew Colville (@mattcolville) March 17, 2018
You can buy a castle in France for $12M. The Mona Lisa is valued at over $600M. https://t.co/2bm7ByJIv0
— Christopher Perkins (@ChrisPerkinsDnD) March 17, 2018
Sorry Chris, that’s not a great example.
The Mona Lisa is the single most famous piece of art on the planet, so the price is something of an outlier.
Meanwhile, old castles like you see for sale in France or Scotland are not in working order – they are quite literally ruins, and their sale prices are actually quite a bit lower than what an intact and working fortress would actually cost.
A better comparison would be between a modern military base and a piece of contemporary high end art. Even a small military base is going to cost hundreds of millions of dollars. But a typical high end art piece is barely going to break the million dollar mark – unless it’s a world famous piece, belonging to the highest tiers of art in the entire world.
I think we need more context to better discuss the issue. A pyramid shaped rammed earth keep with a retaining wall and a 2nd floor is far cheaper than a 5 story stone tower with ivory fixtures and a solid stone cistern. Page 106 of the ORIGINAL DMG lists construction costs in detail.