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

1. Starting Price: 200 euros.
2. Condition: The coin has a condition rating of 90 (XF - Extremely Fine), indicating that it is in very good shape. This generally increases its value compared to coins in lower conditions.
3. Auction History: It has been auctioned once in the past and sold on that occasion, suggesting that there is interest in the coin. The average hammer price in the past was 800 euros, which is an important indicator of market value.
4. Demand: Since there are no records of it not selling in the past, this suggests that there is a steady demand for this coin.

Given that the average hammer price is significantly higher than the starting price, and considering the good condition of the coin, an estimated maximum purchase price could be in the range of 800 to 1000 euros, depending on the competition in the auction and the interest from other buyers.

Therefore, an estimated maximum purchase price could be around 900 euros.