is it possible to find out how much a ticket would have cost if purchased a certain number of days ago? in other words, if an airlines' website is showing a price of $X today, is there a way to find out how much the same fare bucket cost 21 days ago?

the question relates to an international booking i made with a travel agent, who agreed to book certain flights no later than 21* days in advance to lock in the best price. but the agent failed to do so, and now the prices they are quoting me have jumped. unfortunately, i do not have any screenshots or printouts of what the prices should have been - the agent told me that prices often fluctuate daily and their system is designed to book the best price before the cutoff.

*(it may be more than 21 days, but i can't remember the details)