Should I always use miles for expensive flights?

Not necessarily. Sometimes a flight is expensive in both cash and miles. Only use miles if the CPM calculation proves you are getting 'outsized' value.

Does it matter if I pay cash or miles for seat selection?

Yes. Some airlines restrict seat selection or earn status miles differently on award tickets. Check the 'fare class' before booking.

How to Decide Between Using Miles or Cash for Flights

Calculate the cents-per-mile (CPM) value by dividing the cash price by the number of miles required, then compare that to the average redemption value for your specific loyalty program. If your CPM is higher than the average, use miles; otherwise, pay cash and save the points for a higher-value redemption.

Find the baseline cash price. Search for your flight on Google Flights to get the actual cash price, including all taxes and fees. Use the 'booked directly with airline' price to compare apples-to-apples.
Calculate the mileage cost. Log into your frequent flyer account and find the total miles required plus the cash component of the award ticket (the taxes/fees).
Run the math. Subtract the award taxes from the cash price. Divide that result by the number of miles, then multiply by 100. This is your CPM. Example: ($600 ticket - $50 fees) / 50,000 miles = 0.011, or 1.1 cents per mile.
Benchmark against your program. Compare your CPM against established averages. Generally, anything below 1.2 cents per mile is a 'poor' use of points for economy; aim for 1.5 cents or higher for international business or first-class redemptions.

Should I always use miles for expensive flights?: Not necessarily. Sometimes a flight is expensive in both cash and miles. Only use miles if the CPM calculation proves you are getting 'outsized' value.
Does it matter if I pay cash or miles for seat selection?: Yes. Some airlines restrict seat selection or earn status miles differently on award tickets. Check the 'fare class' before booking.