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| Player (age) | From club | To club | Date | Fee | |||
|---|---|---|---|---|---|---|---|
| Show/hide all additional info | Fees in euro (EUR) | ||||||
| 1 |  |  J. St. Juste (22) |  Feyenoord |  Mainz | 2019, Aug | 9 000 000 | |
| 2 | |  J. Mateta (21) |  Lyon | Mainz | 2018, Jul | 8 000 000 | |
| 3 | |  E. Fernandes (23) |  West Ham | Mainz | 2019, Jul | 7 500 000 | |
| 4 | |  P. Kunde (23) |  Atlético | Mainz | 2018, Jul | 7 500 000 | |
| 5 | | L. Ajorque (28) |  Racing Strasbourg | Mainz | 2023, Jan | 6 000 000 | |
| 6 | | A. Fulgini (25) |  Angers | Mainz | 2022, Jul | 6 000 000 | |
| 7 | |  A. Caricol (22) |  Espanyol | Mainz | 2019, Jul | 6 000 000 | |
| 8 | |  M. Niakhaté (22) |  Metz | Mainz | 2018, Jul | 6 000 000 | |
| 9 | | R. Pierre-Gabriel (21) |  Monaco | Mainz | 2019, Jul | 5 500 000 | |
| 10 | |  T. Krauß (22) |  RB Leipzig | Mainz | 2023, Jul | 5 000 000 | |