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

1. Starting price: 40 euros. This is the initial price, but it does not necessarily indicate the final value.
2. Auction history: The coin has been auctioned once in the past and sold on that occasion, indicating that there is interest in it.
3. Average starting price: 86.67 euros. This is a good indicator of the value that buyers are willing to pay in similar auctions.
4. Average hammer price: 475 euros. This is a crucial piece of information, as it shows the average price at which similar coins have sold in the past.
5. Condition: The coin has a condition rating of 90 (extremely fine), which suggests that it is in very good condition, potentially increasing its value.

Given that the average hammer price is significantly higher than the starting price and considering the good condition of the coin, it can be estimated that the maximum purchase price could be in a higher range.

Taking all these factors into account, an estimated maximum purchase price could be between 400 and 500 euros, aligning with the average hammer price of similar coins. However, since the market can be variable, it is advisable to stay attentive to the dynamics of the auction and the competition.