Based on the provided data, here is an analysis to estimate a maximum purchase price for the coin in question:

1. Starting Price: 150 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 25 (BC - Below Circulation), indicating that it is not in very good condition. This generally negatively affects the value of the coin.
3. Auction History: The coin has been auctioned once in the past but did not sell, suggesting that interest or demand for this particular coin may be low.
4. Average Hammer Price: There is no data on previous sales, making it difficult to establish a reference price based on past transactions.

Given that the condition is low and there have been no previous sales, it is likely that collector interest is limited. Therefore, an estimated maximum purchase price could be below the starting price.

Estimation: You might consider a maximum purchase price in the range of 80 to 100 euros, depending on the demand at the auction and the interest from collectors. This would allow for a competitive bid without exceeding the value that the coin's condition might justify.