I think 0.75p for me is generous. I would probably pick a value between .4p or .5p per avios, and would generally not spend more to earn/obtain them as I would feel I am potentially paying more than I am benefiting.

Shorthaul flights in Western Europe cost 9000 avios return + £27(£22ish if you Eurocheat). I would value as a good deal on shorthaul return flight to places like Berlin or Milan from London, at about £75. Therefore if 9000 avios + £27 gives you flights worth about £75, that means 9000 avios are worth £48, or .52p an avios(.47p if you Eurocheat).

Longhaul TATL flights to the west coast in J cost 100k avios return plus about £600 in taxes. I personally would be unlikely to pay more than £1100 cash to fly J on such a route(about £250 extra each way over economy). That means the 100k avios are worth about £500 or .5p an avios on long haul at my valuation.

Obviously my valuations are on the stingy side and there are many other ways to use them, MFU, companion tickets etc. that alter their value depending how used. And of course the value you place on them is completely personal: simply put, they are worth what they are worth to you. That said for me about .4p/.5p an avios seems appropriate.

I obtain most of my avios through Tesco (as well as credit card offers(Lloyds/Amex) etc.) and I will never pay more than 1p per clubcard point, usually less, which works out at maximum .41p per avios.

In summary I don't think .75p per avios is a ridiculous valuation, I just think if you were spending that much to obtain them, you are not really getting that great a deal, especially when you take into account the effort involved to get them. But as I said, it is completely personal...different people's need and use of avios may make for a completely different valuation.