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

1. Current Condition: The coin has a condition rating of 90 (XF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Auction History:
- The average hammer price from previous auctions is 767.86 euros, suggesting that, on average, coins of this reference sell for a considerably higher price than the starting price.
- In previous auctions, hammer prices have varied, but there are examples of sales exceeding 1000 euros, especially for coins in good condition.

3. Starting Price and Average Hammer Price:
- The current starting price is 400 euros, which is reasonable given the history of hammer prices.
- The average hammer price of 767.86 euros suggests that buyers are willing to pay more than the starting price.

4. Recent Trends: In the most recent auctions, hammer prices have been 750 euros and 900 euros for coins with similar conditions (85 and 80).

Based on these factors, an estimated maximum purchase price could be in the range of 800 to 1000 euros, considering that the condition of 90 (XF) is a positive factor and that the auction price history supports this estimate.

Therefore, a reasonable maximum purchase price could be 900 euros.