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 will negatively affect its value compared to coins in better condition.

2. Auction History:
- The average starting price for similar coins is 135.53 euros, and the average hammer price is 306.31 euros.
- In the past, coins with a condition rating of 70 have reached hammer prices of up to 950 euros. However, these coins are in much better condition than the one we are evaluating.

3. Sales Trends:
- Out of the 58 times this coin has been auctioned, it has sold on 46 occasions, indicating good demand.
- The current starting price is 20 euros, which is relatively low compared to the average starting price of other auctions.

4. Previous Prices:
- The hammer prices for coins with similar conditions (BC) have been significantly lower. For example, in previous auctions, hammer prices for coins in similar conditions have ranged from 20 to 70 euros.

Given all this, and considering that the coin's condition is a critical factor limiting its value, an estimated maximum purchase price could be in the range of 30 to 50 euros. This is based on the fact that, although there is a history of higher prices for coins in better condition, the current condition of this coin limits its appeal to collectors.

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