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

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

2. Starting price: The starting price is 400 euros. This is the baseline for the auction.

3. Price history: By looking at previous auction prices for coins with a similar condition (90), we can see that they have reached significantly higher prices in the past. For example, in previous auctions, coins with a condition of 90 have reached hammer prices of up to 800 euros.

4. Average hammer price: The average hammer price for this coin is 231.8 euros, indicating that, in general, coins of this reference tend to sell for a price lower than the starting price.

5. Recent trends: In the most recent auctions, hammer prices have shown a trend of increasing, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price for this coin could be between 600 and 800 euros, considering that the condition is high and there is a price history that supports this estimate. However, since the starting price is 400 euros, it is likely that the hammer price will not significantly exceed this range unless there is strong competition in the auction.

Therefore, a reasonable maximum purchase price could be 700 euros.