Along the Grand Canal

How can I buy an exit?

You won't have what it takes until you've been to several different parts of the story.
You need to have been to Venus...
...and gotten the coin from the Vizicomm Booth.
However, the proprietor will only accept a one marsmid coin.
You also need to have been to the South Pole...
...and "traded" coins with the penguins.
Now just give the one marsmid coin to the proprietor.
When he drops the tube into the dust, simply search the dust.
The exit is inside the tube! It's a portable black circle! It must be on the ground to function.