I think every level increases the training cost by 10k coins. Level 4 costs 40k coins and so i assume Level 5 will cost 50k coins and so forth. Thus to train up to Level 8, it will take around 350k coins? Maybe I'm wrong but I'm sure the stats and coins put in will be better than any elite C cards.
Well as an 80 overall (to better compare to other 80 overall C's) he has a max of 1205 combined secondary stats (probably less because his 78 has +3 in 12 of the the 16 secondary stats and only +2 in the other 4 spots) while Cousins has 1208 and Davis has 1227 but as an 82 compared to 80s he'll have better stats for sure