I am certian of at the moment, academy players gain an even number of stats in every rank per day (there is a slight variance per day of 0.02-0.04 in total stats) and the amount is souley dependent on the investment.
The 2nd thing is the cost effectiveness of training goes significantly down the more you invest into training. For example, if you invest $10'000 per day, it is only 31% as cost effective as investing just $100, however the gains per day are higher. To reach 100 stats from 0 when investing just $100 a day, it would take 4 and a half years and $165'000, whilst investing $10'000 would take 16 months and $5 million.