I guess this is gonna be an endless discussion but it's a good one and I like the way to wrote it, especially focusing on some key points.

A) What players you are aiming for? As in high 5 stars, low 4 stars with SA, young or old players, etc....
B) Which system you are using the most? Auctions, scout, negotiations or recommended
C) How much you are willing to pay for them?
D) How much you often end up paying for them?
E) How much tokens do you spend per season this way.

A - I guess it goes on manager budget and it's kinda a general rule especially now. Those who afford to play with 400, 500 tokens monthly would go more likely to Scout and Negotiation rather then transfer market as prices can be pretty much the same or in some cases even much higher. Those who play with daily tokens would prob. focus on regular players, not to old, not to young, medium quality reported to ongoing star. Recommended finally found it's place with this update for transfer market as you can find good gems there for a good price.
B - Scout (combined more or less with the rest of them)
C - I always had this thing that I should go till the last token if is needed, just for the name but now it's kinda chaotic and in general I try to avoid the transfer market.
D - As much as needed
E - Depends, can be 0 or can be few hundred.