To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Starting price: 5000 euros.
2. Current grade: 75 (MS), which is an average grade.
3. Auction history:
- In 2021, a coin with a grade of 80 sold for 4025 euros.
- In 2024, a coin with a grade of 90 sold for 10000 euros, with a starting price of 3000 euros.

4. Average hammer price: 6425 euros, indicating that, generally, coins of this reference tend to sell above their starting price.

Given that the current grade of the coin is lower than that of the coins that have sold in the past (80 and 90), it is reasonable to assume that the hammer price could be below the average of 6425 euros, but it could still be significant.

Taking into account the starting price of 5000 euros and the price history, an estimated maximum purchase price could be positioned between the starting price and the average hammer price, adjusted for the grade.

Estimated maximum purchase price: 6000 euros.

This price reflects a balance between the starting price and the potential hammer price, considering the current grade of the coin.