Re flights;

1. Skyscanner may and also may not be the cheapest option for flights.
Many of these flight search engines are sponsored. And sometimes Skyscanner may be cheaper,

2. Try Google flights, as yet they are not sponsored by any airline.
Clear your cookie cache memory as they latch onto the fact that you are searching for flights.

3. If you really want a good deal use matrix ita software as its a trade site and is not user-friendly, but it will give you the absolute cheapest deals. I have pasted the link below.

3. Currency fluctuations are a method whereby airlines also make money from passengers, for example, say you specify you want to pay in Australian Dollars, it will give you a price say $x, if you want to pay in Euro it will give you another price, you can work out what is cheaper by using Oanda. I have used this here in Europe, for example it was cheaper by around 10 Euro to book a flight using Ryanairs Bulgarian website rather than their UK website. Its a process called Dynamic Currency Conversion and they all make money at your and my expense.

4. I, like you use spreadsheets to work out spending, try it yourself and do a comparison. The differences may not be spectacular, but the money is better off in your pocket rather than an airlines.

5. Pre Covid I travelled extensively from UK to AUS, HK, China, India and I keep a spreadsheet of every single penny I spend ( I am Scottish) and Google Flights has consistently been the best.

https://matrix.itasoftware.com
Hope this helps, be kind and be safe, kind regards.