I think there is a lot of risk to it. What if you have a horse for one month, its doing well and it gets hurt. Let say it blows a tendon and may or may not be sound again. It requires a minimum of 6 months stall rest before you even know if its going to be sound. That puts six months of stall rest and it may not be sound what do you do with the horse than? If you are counting on 4000 that just tossed that out the window. Likewise, what if a horse is not a good prospect or comes along slower.
I think if you have skill as a trainer and you don't own your own boarding/training facilities I think its easier and better to train horses that are owned by someone else. You don't have as much of the financial risk that way.
In the end, there is money in horses I know I put it there. Which is a nice way to say that horses and training don't make a great way to make money. If you are a student and don't have a stable income than I don't think horses are a way to bridge that gap.