Hi salut0, the price is not bumped up. My subscription ran out in February and I have ever since receiving renewing letter from them. The price (without Avios) is the same as the Avios offer. For example, the offer for Digital and Printing service from my renewing letter without Avios are:
£179 one year/£315 two years/£425 three years/£130 auto renew yearly.
So you can see they did not bump up the price just for Avios. And they did not stop you to renew trough Avios even I am a renewing (not new) customer. This is very good for them.
One thing you have to bear in mind is that the magazine has a different price for different regions. For example, the current British retail price is £5.99 per issue (that is £305.49 for one year 51 issues). £179 or £145 represent a 41% or 52.5% discount.
I believe in the USA the retail price is $6.99, naturally cheaper than the UK. Thus the annual price at $152 (57% off) or $158 (55% off) from Amazon is very cheap.
If you dig deeper in the USA you can get the magazine for a year at the price of USD$90 or so. But in the UK you are less likely to find such bargain price.
And Amazon in UK and US offers different price too. For Amazon USA, the Kindle price for the Economist is $12.99 per month. And in UK the price is £13.99 per month.
So, what is my point? You can not use US pricing to say that the Economist offer in UK has been inflated. It is what it is in UK. we naturally value the knowledge much more than the US. Whether it is horizontal or vertical comparison, I see no foul playing from the Economist in Avios offering in the UK market. But you sir, may be should visit some bargain magazine sites to get the $80-90 offer rather than paying $150 to Amazon.