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

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

2. Previous auction prices: Looking at the auction prices from previous sales, the coin has reached prices of 90 (in January 2022) and 100 (in April 2024) under similar conditions. This suggests that the market is willing to pay high prices for coins in good condition.

3. Average auction price: The average auction price for coins of this reference is 69.4 euros, which provides a baseline for the expected value.

4. Auction trends: The coin has been auctioned 8 times, of which 6 have sold. This indicates a consistent interest in the coin, which may influence its price.

5. Starting price: The starting price is 50 euros, which may be a good indicator of the seller's expectations, but the final price could be significantly higher.

Given that the coin is in good condition and has reached high auction prices in the past, an estimated maximum purchase price could be in the range of 90 to 110 euros, considering that the highest auction price in the past was 100 euros.

Therefore, a reasonable estimated maximum purchase price could be 100 euros.