To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, especially the previous hammer prices and the current starting price.

1. Current starting price: 400 euros.
2. Previous hammer prices:
- The highest hammer prices in previous auctions for similar coins have been up to 700 euros (in the auction on March 14, 2024).
- However, the majority of hammer prices have been between 300 and 400 euros.

3. Average hammer price: 297.25 euros, indicating that the hammer price usually falls below the starting price.

4. Condition of the coin: There is no information about the condition of this coin, which may affect its value. However, in previous auctions, coins with a condition rating of 85 to 95 have reached hammer prices between 300 and 400 euros.

5. Recent trends: In the most recent auctions, hammer prices have shown a trend of being in the range of 400 to 500 euros, especially for coins in good condition.

### Estimation of the maximum purchase price:
Given that the starting price is 400 euros and considering that previous hammer prices for similar coins have reached up to 700 euros, but that the majority have sold in a lower range, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 500 euros.

This price reflects a balance between the current starting price and previous hammer prices, taking into account the lack of information about the condition of the coin.