Your flaw is the comparison of HBD to real-life. I would say its pretty hard to assign arbitrary number values to MLB coaches so you cannot parallel the two. The other flaw is the two monetary systems are different. In MLB the contracts of players and coaches are controlled by what a team can spend based on the income they generate. In HBD, everyone starts with the same amount of money, and the amount spent is dictated by the market. So if you want a 68+ FI coach and only 3 exist, then spending 5M may be required. If you are happy with a 58 FI coach and spending 700K to get him, then budget accordingly. Lastly, comparing the HC/PC to FI is an unfair comparison. Again market availability determines the contract value of both.
I would also argue that a coach that affects your entire organization should be paid more than a coach that only affects one aspect, of one level. i.e. ML PC vs FI. In MLB, such a thing doesn't exist to my knowledge. Each level has their own coaches and within that level, you have coaches that handle fielding, not a "fielding czar" for the entire organization