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

1. Coin Condition: The coin has a condition rating of 25 (BC), indicating that it is in a fairly deteriorated state. This generally reduces its value compared to coins in better condition.

2. Historical Auction Data: By observing the hammer prices of similar coins in the past, we can see that hammer prices vary significantly depending on the condition. For example, coins with higher grades (such as 80 or 85) have reached hammer prices of up to 150 euros or more.

3. Average Hammer Price: The average hammer price for this coin is 86.89 euros, which gives us a reference for what buyers are willing to pay on average.

4. Starting Price and Previous Prices: The current starting price is 40 euros, and the average starting price in the past has been 55.58 euros. This suggests that the market expects the coin could sell for a bit more than the starting price.

Given that the coin's condition is low (25), it is reasonable to adjust the estimated price downward. Considering all these factors, an estimated maximum purchase price could be between 40 and 60 euros, with 60 euros being a reasonable upper limit given the auction price history and the condition of the coin.

Therefore, the estimated maximum purchase price for this coin could be 60 euros.