To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Condition: The coin has a condition rating of 25 (BC), indicating that it is not in the best shape. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- It has been auctioned 6 times in the past, of which it has sold 3 times. This suggests that there is moderate interest in the coin, but also a 50% chance that it may not sell.
- Previous hammer prices for coins with a similar condition (20) have been quite low, with an average hammer price of 6 euros.

3. Starting Price and Previous Prices:
- The current starting price is 30 euros, which is significantly higher than previous hammer prices. This may indicate that the seller expects an increase in interest or value for the coin, but it could also be an unrealistic price given the sales history.
- The average hammer price for similar coins is 8.67 euros, suggesting that the current starting price is quite high.

4. Price Trends:
- In previous auctions, hammer prices have been low, and while there are a couple of coins with conditions of 25 and 30 that will be auctioned in the future, there is no price data for those auctions.

Based on these factors, an estimated maximum purchase price could be in the range of 8 to 12 euros, considering that the starting price is 30 euros and that the hammer price history is low. This reflects a more realistic offer given the condition of the coin and the moderate interest in the past.