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 not in very good shape. This generally reduces its value compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins in the past has been 51.22 euros.
- Previous auctions show that coins with similar conditions (some with a condition rating of 30) have reached hammer prices of up to 33 euros.
- The most recent auction of a coin with a condition rating of 30 had a hammer price of 30 euros.

3. Starting Price and Previous Sales:
- The current starting price is 10 euros, which is relatively low and may attract more bidders.
- The sell-through rate in previous auctions is high (18 out of 20), suggesting there is consistent interest in this coin.

4. Price Trends:
- The average starting price of 39.9 euros and the average hammer price of 51.22 euros indicate that, in general, bidders are willing to pay more than the starting price.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 30-35 euros. This is based on the hammer price history for coins of similar condition and the demonstrated interest in previous auctions.