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

1. Current Condition: The coin has a condition of 25 (BC), which indicates that it is in quite low condition. This will negatively affect its value compared to coins in better condition.

2. Historical Prices: By looking at the auction prices of similar coins in the past, we can see that prices have varied considerably. For example:
- In 2019, a coin with a condition of 100 sold for 2300 euros.
- In 2022, a coin with a condition of 95 sold for 1200 euros.
- In 2021, a coin with a condition of 70 sold for 970 euros.

3. Starting Price and Average Price: The current starting price is 500 euros, and the average starting price in previous auctions is 1055 euros, with an average hammer price of 1343.11 euros.

4. Sales Trends: The coin has been auctioned 11 times, of which it has sold 10 times, indicating good demand.

Given that the condition of the coin is low (25), the hammer price should not approach the prices of coins in better condition. However, considering the price history and demand, an estimated maximum purchase price could be set at a lower range.

Estimated Maximum Purchase Price: Based on the condition and historical prices, a reasonable maximum price could be around 400 to 600 euros. This aligns with the starting price and the price trends of coins in similar conditions.