[OPEN LOW] Wrong numbers for cost calculation.

When you build on the edge of the map, the wrong costs are displayed.
1 x 5 = 60$ but when you build it, it's 30$
2 x 5 = 30$ but when you build it, it's 60$

Because it's easy to reproduce, there's no save game or log detached.

Screenshots can be found HERE
Sourcelink for screenshots: https://imgur.com/a/VeLcncP

Hi, thanks for catching this! Should be an easy fix, but will at the same time need quite a bit of testing, as the bit of code I'm looking at is widely used throughout building mode. Marking as open/lower priority.