To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting price: 30 euros. This is the initial price of the auction.
2. Auction history: The coin has been auctioned 2 times in the past and has sold on both occasions, indicating consistent interest.
3. Average starting price: 35 euros, suggesting that the current starting price is slightly lower than the historical average.
4. Average hammer price: 51.5 euros, which is the average price at which the coin has sold in previous auctions.
5. Condition: The coin has a condition rating of 25 (BC), indicating that it is not in very good shape. This may negatively affect its value compared to coins in better condition.
6. Previous prices: In the April 2022 auction, the coin had a starting price of 40 euros and sold for 73 euros, showing that there is potential for a higher hammer price, although the condition of that coin was better (condition rating 70).

Given that the condition of the current coin is significantly lower (25), it is reasonable to adjust the estimated price downward.

Taking all these factors into account, an estimated maximum purchase price could be between 50 and 60 euros, considering that interest in the coin remains and that the average hammer price is 51.5 euros, but adjusted for the lower condition.

Therefore, a reasonable maximum purchase price could be 55 euros.