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

1. Current 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. Auction History:
- The average hammer price for similar coins in better condition (for example, with condition ratings of 70 to 95) has been significantly higher. For instance, in previous auctions, coins with condition ratings of 70 and 80 have reached hammer prices of up to 170 euros.
- The last auction of a coin with a similar condition (25) had a hammer price of 30 euros.

3. Starting Price and Average Hammer Price:
- The current starting price is 50 euros, which is lower than the average hammer price of 211.82 euros for coins in better condition. However, given the low condition, it is likely that the actual hammer price will be below this average.

4. Sales Trends: The coin has been auctioned 31 times, of which it has sold 28 times. This indicates a consistent interest in the coin, although the condition may affect demand.

Based on these factors, an estimated maximum purchase price for this coin, considering its condition and price history, could be between 30 and 50 euros. Since the starting price is 50 euros, it is likely that the hammer price will not exceed 50 euros, especially considering that in previous auctions, coins in similar conditions have had hammer prices around 30 euros.

Therefore, a reasonable estimated maximum purchase price could be 40 euros.