As much as we often complain about bloggers here, I think there are some that do a reasonably good job of calculating the rough value of points/miles/avios. I think the general thinking is that an avios is worth around £0.01 or $0.013. Buying them directly will cost more than this. Spending them at a value much below this is not the best use of points but may still be worthwhile depending on your circumstances (having a lot of avios vs. having a lot of cash, etc.).

To me, the best value of Avios is on bookings within 2 weeks of departure, family holidays in premium cabins, or as a way to reduce a hotel rate (sometimes the cash + avios price is much better than booking the hotel outright).