Youth trainer is definitely cheaper and more effective than a gym. The gym is really useful if you're trying to make a NT or U21 player but other than that it's a huge financial sinkhole for what is likely 1 pop per season (if it would be any more we'd see a lot more 160+ TSP players).
Thus I'd say if you're looking for cost effectiveness you want a lvl 5 or 6 trainer, lvl 5 or 6 Youth Trainer, no gym, and I'd say lvl 1 training Court since a lvl 1 is typically fine to get them up near the 20 mark by 26/27.
But yeah as far as available data on this, there isn't much. Most people collecting data have long left BB.
2x NBBA Champion: S55, S56. 3x USA Cup Champion: S54, S55, S56. WR for longest home streak ever at 11 in Season 47.