To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 12 euros.
2. Condition: The coin has a condition rating of 25 (BC), indicating that it is not in very good shape.
3. Auction history: It has been auctioned once before, with a starting price of 12 euros and a hammer price of 12 euros, suggesting that there has not been significant interest above its starting price.
4. Average hammer price: The average hammer price is also 12 euros, reinforcing the idea that interest in this coin is limited.

Given that the condition is low and the auction history shows no price increase, the estimated maximum purchase price could be slightly above the starting price, but not much more, as interest appears to be low.

Therefore, an estimated maximum purchase price could be around 15 euros, considering there might be a slight increase in demand or interest in the current auction.