To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- The coin has been auctioned 5 times and has sold on all occasions, indicating consistent demand.
- The average hammer price in previous auctions is 12,660 euros, suggesting that buyers are willing to pay a price above the starting bid.

3. Previous Prices:
- In the March 2023 auction, a coin with the same condition (90) sold for 16,000 euros.
- In the March 2024 auction, a coin with better condition (95) sold for 16,800 euros.

4. Current Starting Price: The starting price is 10,000 euros, which is reasonable given the history of hammer prices.

Based on this data, an estimated maximum purchase price could be positioned between the average hammer price (12,660 euros) and the highest price reached in previous auctions (16,000 euros).

Therefore, a reasonable maximum purchase price could be approximately 15,000 euros. This reflects the good condition of the coin and the interest demonstrated in previous auctions.