Don't know about your "facts" but my MyGranTurismo page says the following:
Greycap owns 1018 models in his mygranturismo.net garage. Before tuning, the value of his garage reaches 456,937,013 Cr, the total weight of 1,256,256 kg, and the cumulative power 308,003 BHP. By selling all his sellable models, he would earn 16,753,593 Cr.
Looking at the garage value, add to that the around 9 million credits I have now, divide by the amount of victories which stands at 1951 at the moment, and you'll have an average prize of approximately 237,634 Cr. I know that the figure doesn't take into account expensive prize cars but neither does it acknowledge that extra Miura and more than a few duplicate LMPs, for example. The total amount of cars is somewhere past 1100.
My GT4 game save tells me that I have around 9 million credits in the bank, my prize money total is 37,193,500 Cr and that I've received 341 prize cars. Not knowing what those prize cars have been I'll throw out a guess of 500,000 Cr per car which is probably too much but at least it won't be an underestimate. The value of those prize cars thus stands at 170,500,000 Cr and combined to the prize money the total is 216,693,500 Cr. Apparently I've won 1287 races which brings the average income per victory to around 168,371 Cr. I have 490 cars.
Doing the math reveals the average income per race to have been nearly 70,000 Cr higher in GT5. I can't think of many "grindable" races in GT4 that paid the GT5 average, even with the prize cars included. The DTM is actually the only one that comes to mind. Even the hardest series had prizes of around 25,000 Cr per race and a 250,000 Cr overall bonus - 50,000 Cr per race plus the car which was usually kept anyway. Now similar series pay about twice that amount while being half as long so you can do it twice in the same time for quadruple money.