To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting Price: 75 euros. This is the initial price of the auction.
2. Condition: The coin has a condition rating of 30 (BC+), indicating that it is not in the best condition. This may affect its value compared to coins in better condition.
3. Auction History: The coin has been auctioned 4 times and has sold on all occasions. This suggests that there is consistent interest in the coin.
4. Previous Prices:
- In 2016, it sold for 25 euros and 65 euros in two different auctions.
- In 2021, it sold for 75 euros, indicating that the value has been increasing.

5. Average Hammer Price: 60 euros, which is a good indicator of the current market value.

Given that the coin's condition is 30 (BC+), and considering the starting price of 75 euros, the average hammer price of 60 euros, and the price history, an estimated maximum purchase price could be in the range of 60 to 75 euros.

However, since the starting price is 75 euros and considering that interest in the coin seems to be strong, a reasonable maximum purchase price could be approximately 70 euros. This allows for some margin for competition in the auction, but also takes into account the coin's condition.