i think ur cost estimation is a bit not very accurate:
 So to train a SF you will need an 18 yr old trainee, thats going to cost around 1.2mil
with 1.2mil u can buy a top 18 SF prospect with high potential, but for an SF the potential needs not to be an MVP and it cost you much less if u accept that ur trainnee is about 4-5 skills lower than the top ones, which should take u half a season longer to train him to attain the same height
this is my trainnee. his skills are quite gd in my opinions. he only cost me $135000. more importantly, he's from a small country and surely he'll be an U21 with 2-3 seasons training:
Small Forward  
Owner: Red Red Rose
DMI: 9500
Age: 18
Height: 6'6" / 198 cm
Potential: perennial allstar 
Game Shape: average  
Jump Shot: respectable  Jump Range: awful  
Outside Def.: proficient  Handling: average  
Driving: respectable  Passing: pitiful  
Inside Shot: inept  Inside Def.: average  
Rebounding: average  Shot Blocking: pitiful  
Stamina: awful  Free Throw: average      
Experience: atrocious 
You will need a decent trainer...
level 5 - thats going to cost you about 300k up front and 20k per week.
level 6 - 1mil up front and 20k per week.
20k per week over 7 seasons = 20k x 14 weeks x 7 seasons = around 2mil
but for this part, u underestimate the cost. the salary rise of a trainer is large. when u first buy him, he cost u 20k per week. but 7 seasons later, he may cost u 60k-70k per week and u may better buy another trainer...
personally i am for training SFs. coz they sell for a high price. and dont forget u should at least train 3 SFs at the same time. so after 6-7 seasons training, u can sell 2 of them which gains u 8-9 million and keep the best one. then with 8-9 million u can basically buy 4 top players for the other 4 positions and have a championship team.
and for myself, i rejoin this game simply coz i enjoy the process of training  an SF (and the pride to have one as well). and i don't care about staying in division 4 in the next 5-6 seasons ^^