What's the formula for District production cost?

So Districts, like Setllers, Missionaries, and Workers, increase in cost over the course of the game, the more of them you build.

However, it appears the cost of Districts increase in ways other than that, as oftentimes, the very first district I build has a production cost higher than the base cost (as listed in the production queue tooltip).

What factors contribute to the increase in cost?

This isn't documented anywhere as far as I could find. I experimented a bit myself, and I think I found the major factor for district costs:

• the cost increases with the number of technologies and civics discovered
• the increase is the cost of the technology or civic divided by 10
• the first three technologies don't seem to count (pottery, animal husbandry and mining)
• the craftsmanship and foreign trade civics increase district costs by 11 each, more than the 40 culture cost would indicate

There are still some cases where those rules don't explain the cost increase, so I haven't figured it out completely yet.

The following factors don't increase the cost as far as I could observe in my experiment:

• turn number
• number of cities
• number of districts built

The cost of a district is locked once you started building it, it doesn't increase while you're building it.