I echo the above. Nothing is clear cut anymore in a cash v redemption scenario so you cant ever assume booking one particular way is always going to be cheaper. Also keeping in mind cheapest doesn't necessarily equal best value depending on your own needs for the reservation.

Cash fares with some avios thrown at the situation to reduce the cost will still earn avios and tier points but are ultimately still going to be governed by the underlying fare rules and restrictions of the fare being purchased. I may be in need of some Avios or tier points or indeed need the sectors from a cash fare to go towards my elite status.

Avios redemptions with a varying cash element aren't going to earn me avios or tier points or count towards status but the flexibility to cancel until 24hrs prior to travel may be useful to me. Also the fact 1 piece of checked baggage is included may also be a differentiator when flying Economy.

Sometimes redemptions are a non brainer For example BA Club World Buenos Aires - Rio for redeeming 17750 avios + £25 v paying £791 cash and earning 40 tier and circa 5600 avios points stacked up for me. Other times it's less clear cut so making the comparisons is always recommended so that you can make the right decision based on your own particular needs

Generally speaking I usually have a figure in mind that once a cash fare is more than X, it's probably going to be better value to book it as a redemption booking. Unless I'm in dire need of the avios or tier points.

Redemption space is always limited, not every seat can be booked as an avios redemption so once those options disappear on any given flight then all you've got left is cash options.