So, at target, Tomato Sauce is $0.84 / lb. But, tomato paste is $1.00 / lb. However, tomato paste has 4x as many calories, implying that they reduced it by 4x. This makes sense, since reducing tomato sauce to tomato paste in a pot generally reduces it by 4x. But, why is it not 4x as expensive? How is it only barely more expensive if it requires so many more tomatoes to create? Generally speaking even boiling for longer costs money. Milk powder is almost always more expensive than milk, for example (In all of the grocery stores that I've checked, at least). I can't find tomatoes for less than $1 / lb, which upsets me because I want to homemade tomato sauce and tomato paste but it's just not cost effective. Where are they getting practically free tomatoes?
What you are observing is that the bulk of the price of tomato products is not the tomatoes.
The price of the can and shipping far outweigh the few cents of tomatoes the can contains.
With fresh tomatoes you are still mostly not paying for tomatoes. You are paying for the gas and people to ship and stock your tomato and all the other tomatoes that were shipped with yours and thrown away damaged or unsold.
It's cheap because it is easy and cheap to make/produce.
They don't need to have perfect tomatoes to make tomato paste, compared to making canned tomatoes or tomato sauce. (IMO)
one run at Google shows a more complex process to make tomato sauce. https://image.slidesharecdn.com/rhidimaandvasudha-150515065227-lva1-app6892/95/tomato-processing-7-638.jpg?cb=1514799842
but i guess this answer would depend on recipe for said product. So the real question should be 'Why is tomato sauce not 1/4th the price of tomato paste?'. The answer should be 'There is increased cost due to other ingredients added to the tomato sauce in processing.'