ILLS, I'm confused. I've clearly demonstrated how to calculate payouts but you seem skeptical of the math. It's very straightforward. Suggesting that an anecdote is a "better" answer than the actual math is strange to me.
I went to that thread you pointed to, saw that the one guy used 528PP, and without looking at what he won, I used a piece of scratch paper to estimate the multiplier was around 6.1x. Added that to the 200% for login bonus, multiply that by 550,600 and you get EXACTLY what the guy won.
In case you are curious where the 6.1 came from:
650PP --> 0x
610PP --> 2x (As stated on the race page)
570PP --> 4x
530PP --> 6x
490PP --> 8x
528PP is very close to 530PP, so I estimated 6.1x.
The math is (550600 * 200% login bonus) + (550600 * 6.1) = 4459860.
How can I explain it any clearer? It works for every race, for every PP, for every place you finish. Please note, at certain times, GT5 changes its payouts, even for previous races - to reward us or something. The math I've shown can be used regardless if they change or not; whereas the anecdotes won't apply to new payouts or to different races.