To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Coin Condition: The coin has a condition rating of 90 (EBC), indicating that it is in good shape. This generally translates to a higher value compared to coins with lower conditions.

2. Auction History:
- It has been auctioned 8 times in the past, of which it has sold 6 times.
- The average hammer price in previous auctions is 41.25 euros.
- In the most recent auction (2023-05-23), the hammer price was 26 euros for a coin with a condition of 95, suggesting that the price may vary depending on the condition.

3. Starting and Estimated Prices:
- The current starting price is 30 euros, and the average starting price in previous auctions is 25.14 euros.
- The estimated price for this auction is 30 euros, which matches the starting price.

4. Price Trends:
- In the December 2021 auction, a coin with a condition of 90 sold for 60 euros, indicating that there is high selling potential for coins in good condition.
- In other auctions, hammer prices have varied, but the range seems to be between 24 and 60 euros.

Based on this data, an estimated maximum purchase price could be set in a range between 50 and 60 euros, considering the good condition of the coin and the historical hammer price data from previous auctions. This would allow for competitive bidding without exceeding a reasonable limit based on historical performance.

Therefore, an estimated maximum purchase price could be 55 euros.