Viewed as a standalone purchase it is truly grim value yes, but it's the minimum amount you can buy so it would always be thus (if you were buying 10,000 you'd be paying just over half the amount per Avios)

I'd suggest instead view it as part of the total cost. A common nominal value on an Avios is 1p, so if you turn it around and say your £31 would be worth 3,100 more Avios, would you still have done the upgrade to F if the quoted price had been 3,100 more Avios ? If yes, it's still good value to you.

The only other thing I can suggest is that if you know anyone who might transfer some unneeded Avios to you, you could try that route, but unless they are Gold or above I believe that costs money, and do you want to risk the seats going? Personally I'd book now as you are doing