I would like to know what the RT cost would have been to fly from CMI to MSP on July 30, 2021. Return on August 3, 2021.
The simple answer is that there's no way to find out.
The price of flights varies not just based on the dates of the flights, but also multiple factors including :
- How far in advance of travel the tickets were booked
- How many seats were available at the time of booking
- The exact date the flights were booked, in terms of any fare sales/etc occurring at the time.
- The country that the tickets were booked in
- The exact route for the flight (there do not appear to be any direct flights between CMI and MSP)
and probably numerous other factors.
Even if you were to find someone that flew that exact route on those exact days, and asked them what they paid, it would not be a good indication as it still wouldn't cover most of the above points - in particular when the ticket was purchased.
You can get a very (very!) rough estimate for the price by looking at a similar flight sometime in the future. eg, October 22->27 (Friday -> Wednesday as per your original dates) shows prices ranging from $317 up to $547 for a 1-stop trip. This MIGHT give an indication of a typical price for such a trip booked 3+ weeks in advance - or it might not as pricing can vary depending on the exact days (eg, school holidays, events occurring in the relevant towns increasing demand, etc, etc), sales occurring at the time of booking, etc.