I didn't want to write all that so I waited for dedelman and brianplath.

Agreed 100%.
IMO Coach Hiring is nothing but a chore and an overspend if you treat it as competing for the best free agents. I rehire from within as much as possible, promote them whenever possible.
Keep a good FI as long as you can. First thing I check after rollover (I'm not kidding) is whether my FI is coming back. If not it's an automatic $3-4 mil add to my coaching budget - I want to make sure I get whichever of the top three FIs available are, because all the other owners are bringing theirs back if they can. If you're in the market for a new FI you're already at a budgetary disadvantage.
The flaw in the system IMO is that whenever coaches reach their core rating (Pitching, Hitting, Strategy) at 75/80+, they start looking for Major League jobs.
In theory I guess it makes some kind of sense but in practice it creates a huge backlog of coaches for those roles at the ML level. They're not getting hired because like brianplath inferred, you can just wait and get an 80-85ish coach for whatever pocket change you have left.
Also, this is just me reading into it, but I believe this threshold tells me that the game thinks it's perfectly fine to toodle along with Major League coaches at the 75/80 promotion level. ie Replacement Level / Won't hurt your players.
All these things in play, the only time I ever spend the full $2.2-$2.5 mil ask for Major League coaches is if I'm keeping my current ones because I have an (illogical-ish) attachment to them. Especially the Bench coach, biggest waste of money in the entire game. Admittedly I'm wasting 2 mil on my own right now.