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

1. Condition of the coin: The coin to be auctioned has a condition 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. Price history: By looking at the hammer prices of coins with similar conditions in the past, we can see that prices have varied. For example:
- In recent auctions, coins with a condition of 85 have reached hammer prices of up to 360 euros.
- Coins with a condition of 90 have reached hammer prices of up to 400 euros.

3. Starting price and average hammer price: The starting price for this coin is 400 euros, and the average hammer price in the past has been 345.76 euros. This suggests that the starting price is reasonable, but it also indicates that there could be room for the hammer price to be higher, especially if there is interest in the coin.

4. Recent trends: In the most recent auctions, hammer prices have shown an upward trend, especially for coins in good condition. This suggests that there may be growing interest in this coin.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 450-500 euros. This is based on the hammer price history for coins of similar condition and the potential interest in the coin.