"if I ask a mate to take me up in their aircraft for a spin I would be paying a lot more than 50%."
Too damned right!!! It would probably be your life!!
Why not? Good fun, I have spun aircraft a few times - as long they are certified for spins and you have the relevant training & are competent go for it!
On the other hand, I remember reading somewhere that if you were in the USA and have an FAA PPL then you are allowed to fly around an MP (or Senators as they call them) and recieve payment. I dont remember the full details but hoping someone Stateside can come up with facts.