The only time I pay more than 0.5p is when there is really no work involved ... such as BA offering me Part Miles Part Cash where the net cost is 0.75/0.8p, or when I pay tax on my '2 miles per £1' BMI Mastercard for a 1.4% fee and thus pay 0.7p per mile.
I agree, if the avios are obtainable at almost no effort, then i will pay a bit more to get them.
Although this debate is becoming a bit pointless for me, because we will be over 2m Avios after the BMI conversion and the Lloyds Amex is put to bed, and at that level I'm not sure that any price is worth paying for more!
The problem is that collecting them is so addictive!