1. Canada didn't pay market price because there was no market. A market price exists when consumers can go out and pay money to buy something, which they could not do with the vaccine. While lacking a market price, we do have an idea how much consumers would be willing to pay if they had the option to buy.
2. The value of the vaccine to consumers in the Western countries was multiple thousands of dollars. My wife and I would have spent 5 figures to get the vaccine when it first became available but had to wait our turn. India exported 57 million vaccine doses to the world (not just Canada), and if you put a low market price of say $2,000 per dose, it would have been $114 billion. Total contributions to Covax which is buying vaccines mainly produced in India is less than $3.2 billion as of April 2021.
https://en.wikipedia.org/wiki/COVAX
3. You don't understand the process of vaccine manufacture. There is a vaccine recipe and vaccine production capacity, and the scarce factor has the power. The scarce factor is vaccine production capacity.
Suppose AstraZ and Serum Institute of India could not reach a deal, who would have lost more? It would be AstraZ because it can't replace the production capacity of SII. SII can however replace Astra with a vaccine recipe from Sputnik or Bharat Biotech or J&J or Baylor College of Medicine (which is collaborating with the Indian firm Biological E) etc.
Also, Astra can sign all the contracts it wants, but the Indian government has final say about approving export of vaccines manufactured in India.