Ive looked up the build formula but cant seem to get an answer that makes sense. Anyone able to show me the math

BuildCost = 3*land+1500

basically if you have 20k acres of land then 1 building costs 61500 bucks.

in your case 3*7600+1500=24300 per building so with 80 bpt you need almost 2 million bucks to be able to build 1 turn.

On the earth formula page that formula is crossed off...is it still correct?

that is latest, non-crossed was reverted to that.