To estimate a maximum purchase price for the coin in question, let’s consider the following factors:

1. Starting Price: 500 euros. This is the initial price set for the auction.
2. Auction History: The coin has been auctioned 2 times in the past, of which it has sold 1 time. This indicates moderate interest in the coin.
3. Average Starting and Hammer Price: The average starting price in the past is 270 euros, and the average hammer price is 650 euros. This suggests that, although the current starting price is higher, interest in the coin may lead to a higher hammer price.
4. Condition: There is no information about the current condition of the coin, which may make bidders more cautious. However, the previous hammer price of 70 euros for a coin with a condition of 20 suggests that interest may vary depending on the condition.
5. Previous Prices: In the last auction, the starting price was 40 euros, indicating that interest and the value of the coin have increased over time.

Given that the current starting price is 500 euros, which is significantly higher than the previous average starting price, and considering that the average hammer price is 650 euros, we could estimate that the maximum purchase price could be in a range between the starting price and the average hammer price.

Taking all these factors into account, an estimated maximum purchase price could be approximately 600 euros. This is based on historical interest and the potential for bidding, but it also considers the lack of information about the current condition, which could limit bidder interest.