I can only say that L5 trainer (you can get one with around 20k starting salary for around 500k I guess) can bring a solid rookie in 55-60 TSP range to 130+ TSP at the end of his 28 years.
You can GET a Level 5 trainer for 30k starting price
Whether you sell him somewhere along the line or not is another decision. I usually keep them until 34 or 35 when they are not worth anything anymore, so I can't say I made any profit of training them. But they surely deliver in the meantime.
You sale the player when he makes the National team farm and at the highest peak of the national team. You also sale them 28-32 for the people who think they are good.
It depends on what you prefer, looking for profit (and trying to build your team with the money you get) or building players that suit you. For me the answer is simple, there are other ways to earn money anyway.
Profit is made in long term planning and training schemes.