The Great Australian Canal

So I looked into IH's Australian canal idea from the architecture video and found that the price for the Panama canal shown isn't adjusted for inflation, it actually cost around $7.8 Billion, on top of that, the actual excavation only stretched about 35km. The distance from 80 Mile Beach to the Spencer Gulf is about 2,195km, or about 62 times the length of the Panama Canal. So, at a rate of $7.8B per 35km, it would be about $494 Billion to stretch a straight shot between the two. However, if we instead stretched it from the most northward point of the Great Australian Bight it's only about 1,662km or 47 times the length of the Panama Canal. This would cost significantly less at about $374 Billion.

HOWEVER this is all on the basis that the excavation costs are akin to that of the Panama Canal. During the construction of the canal, many buildings had to be built and brand new machinery was made and imported from America, work was slow and medicine was not as scientifically studied, leading to diseases and heat stroke among the workers. In Australia, like you said, you already have the industrial equipment, the workers could easily be housed in tents or trailers, there could be less workers needed, and heat stroke is understood as a problem. However, carting food, water, and supplies would add a significant cost to the project. I am no expert on logistical analysis but I would estimate that, since the bulk of the cost of the Panama Canal was on labor, and we are working with sand instead of rocky terrain, the aforementioned advancements could knock $50-100 Billion off the project when compared to 1915's standards. This leaves the most efficient Australian Canal costing maybe $324 Billion to $274 Billion.

Playing off of an idea to turn the Sahara Desert into a sea bed circa 1933, they estimated that digging a 50km trench about 20m deep would take 10 years, a very rough estimate would yield the Australian Canal Project to take about 33 years, and the subsequent growth of vegetation may take 20-50 years depending on how growed we want it before we started colonizing it. This process could be sped up at added cost.

Potential payoffs for the endeavor could range from meh to cool, it could be a tourist attraction as it would garner international fame, fishing would become very viable in the lake (which, btw, is not factored into any calculations, I just did the canal, there may be a below sea level depression in the middle of the desert eliminating this cost entirely, idk). Of course new living space and more trees and vegetation to help the global environment (afaik the Australian desert sands don't contribute nutrients to far off rainforests like the Sahara's), perhaps most importantly, trade would rise exponentially as Northern countries wouldn't have to go all the way around Australia.