By power training and farming Green/Red packs during the training, as well as playing friendly and competitive matches I believe I could get a full roster of fast young players to 9* for under 3K Tokens total.
It would probably take me the best part of a week to do it and would be a lot of work, but I think its doable. Only when you use personal trainers does it start to get expensive Token wise.
The question I would ask is why anyone would go to the trouble of having that many 9* players. The game would be really boring if you are hitting every team for 6/7 every game and every player you had dwarfed your nearest competition. It would be like going to the local primary school and joining in a game of football with a bunch of 6 year olds. Yeah, you'd be better than them and win, but it would a shallow victory overall.
I personally like to be competitive, but don't want to have a way stronger team. I've over 10K tokens in my account this season, yet I've got a team that has just hit 6* average today - and I'm the second strongest team in my league. I could easily push my players to all be 7 or 8 star and use a load of my tokens but what is the point? I will get promoted at the end of this season, so my end goal will still be the same. I'd rather keep my tokens for when I might need them and have a competitive season against similar matched teams.