Unfortunately with how slow the pops come and without any proper way to know how much effect the gym really has (hard to approximate sublevels + where they go) something like that would be very tough. I will say though, if you are training for u21 or NT gym being at least level 2 is really helpful. Training court level 1 is pretty much a must for any team, level 1 is the best return for cost imo because you still get your players to a solid level without a ton of weekly fees. YT typically runs cheap so I don't skimp on it, but again that may be because I usually train my young guys for u21 and thus need all the edge I can get. Trainer at level 4-5 is optimal bang for buck and definitely the best route unless you are training a generational talent, in which case you may need to upgrade.
That's how I view things anyway. I think most of the others here would probably say similar things.