Its not an easy answer. What I can tell you is some general things to consider from my experience. But know its just one person's opinion.
Prospect Payroll :$ - Are you going to play the IFA market? Do you have a lot of extra high draft picks coming up due to A and B FAs? Might want to pad this.
Coaches Payroll :$ - Is your Fielding Instructor resigning? Are your other BL coaches willing to resign and are cheap? If not, you might want to pad this.
For these -
Domestic College Scouting Dept. :$ M
Domestic High School Scouting Dept. :$ M
Generally, most of the better owners I have talked to pick one or the other. College gives you a bigger pool, but you may have 2/3 of the world focusing on college, so HS might give you a deeper, more talented pool. You need to check recent draft history to figure out which way owners tend to go.
International Scouting Dept. :$ - Are you playing the IFA market? If so, you need to beef it up. If you aren't, then keeping it low or zero is best.
Advance Scouting Dept. :$ This helps you know where a player will - probably - end up. This is only for SIGNED players, not unsigned draft picks or IFAs. Most good owners think this to be useless, but starting out, I maintained a high budget here while i learned how to project prospects. Now I am dialing it back to zero across the board. Its only really useful at $18 or above in my experience.
Training :$ It controls how susceptible players are to injury, and how much they will develop speed and strength in the offseason if they are prospects or young BLers. Should be $18 to $20 million. I don't see how you can really punt on Training.
Medical :$ This controls the recovery of injury and can go either way. Owners like Mike T max it out and when a player gets injured, they can come back better than before. I completely punt on it.
Hope that helps somewhat.