https://forum.paradoxplaza.com/forum/developer-diary/dev-diary-tile-upkeep-explained.1692037/
Of course, this new cost affects your ability to reach the edge of the playable area and create new Outside Connections, so let’s talk briefly about the cheapest way to create them early on. If you haven’t already noticed, map tile costs are affected by what is available in them - buildable land, resources, etc - making tiles with a lot of water the cheapest. This makes buying ocean tiles the absolute cheapest way to go if your city needs that new Outside Connection as soon as possible. You can then create a bridge or pipeline to the edge of the playable area. Or you can use the terrain tools to create a more realistic landbridge and build a road on top of it, but keep in mind that more land inside the tile affects its upkeep cost.