To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, including the starting and hammer prices of previous auctions, as well as the condition of the coin.

1. Current starting price: 400 euros.
2. Average starting price in the past: 228.13 euros.
3. Average hammer price in the past: 306.18 euros.
4. Hammer price history: Hammer prices have varied, with a maximum of 650 euros in 2020 for a coin in condition 95.

### Analysis of previous prices:
- The coin has had a maximum hammer price of 650 euros and a starting price of 300 euros in previous auctions.
- The condition of the coins auctioned in the past varied, and although we do not have specific information about the condition of the current coin, the highest hammer prices correspond to coins in conditions 95 and 90.

### Maximum price estimation:
Given that the starting price is 400 euros and considering that the average hammer price is 306.18 euros, we can assume that the hammer price could be in a range above the average price, especially if the coin is in good condition.

- Range of previous hammer prices:
- If we take into account the highest hammer price (650 euros) and the current starting price (400 euros), we can estimate that a reasonable hammer price could be between 400 and 600 euros, depending on demand and condition.

### Conclusion:
An estimated maximum purchase price for this coin could be 600 euros. This price considers the auction history, the current starting price, and the possibility that the coin is in good condition, although it is not specified.