Even if he becomes an excellent player, a defensive midfielder is not a star. The highest amounts of money are normally paid for match winners, so I can understand a big bet with Martial, but not for a physical defensive midfielder. It would be like spending 30-40 M. on a defensive right back because he looks strong and composed at 18. If he was a complete player in the making I'd see it right, but it's not the case.
To perform a defensive/supporting role all you need is a sober-headed player, with good physical conditions and acceptable technique, there's no need to make flashy business and search any kind of superstar. I don't mean there's hundreds of players who would be good enough for us, but a lot of teams have signed good defensive midfielders in the last years, and they didn't need to get crazy spending.
Wolfsburg paid around 15 M. € to sign Gustavo, and Leicester got Kante on 8 M pounds. Real spent 5 M. on Casemiro and he's found a place there, and PSG paid 6 M for Matuidi some years ago, as they did with Stambouli last summer. I'm sure 10-15 M it's more than enough to sign a good defensive midfielder if you are clever as a club, there's no need to sell our house to add some strenght to the team.
I know I won't pay it directly from my bank account, but I don't think it's healthy for a club to spend 30-40 M on every player like we've been doing recently. Our quality/cost balance is amateurish, and at this rhythm we're going to have the most expensive squad in football history while we play the Europa League.