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

1. Condition: The coin has a condition rating of 95 (S/C-), indicating that it is in very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- In the December 2021 auction, the coin was estimated at 3000 euros and sold for 2000 euros, with a condition of 95.
- In September 2022, the coin was auctioned with a starting price of 1200 euros, an estimated price of 2000 euros, and sold for 1700 euros, with a condition of 90.
- In March 2024, it was estimated at 2500 euros and sold for 2800 euros, also with a condition of 90.

3. Starting Prices and Average Prices: The current starting price is 1800 euros, and the average selling price in the past has been 2166.67 euros.

4. Price Trends: Looking at previous selling prices, there seems to be an upward trend, especially in the last auction where it sold for 2800 euros.

Based on this data, an estimated maximum purchase price could be in the range of 2500 to 3000 euros, considering that the condition is high and interest in the coin appears to be increasing. Therefore, a reasonable maximum purchase price could be 2800 euros.