To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the conditions of the auctioned coins.

1. Current starting price: 400 euros.
2. Average hammer price: 308.5 euros.
3. Auction price history:
- Previous hammer prices varied significantly, with some reaching up to 1100 euros in higher conservation conditions (like 90).
- Most hammer prices fall between 150 and 500 euros, depending on the condition.

4. Condition: No specific information is provided about the condition of the current coin, making the estimation more complicated. However, we can observe that coins with conditions ranging from 70 to 90 have had hammer prices ranging from 200 to 600 euros.

5. Trends:
- Coins with higher conditions (80-90) tend to achieve higher hammer prices, while those with lower conditions (70 or less) have lower hammer prices.
- The coin with a starting price of 400 euros suggests that considerable interest is expected, which could indicate that the coin has a higher perceived value.

### Estimated maximum purchase price:
Given that there is no information about the condition, but considering the starting price and the auction price history, a reasonable maximum purchase price could be between 500 and 600 euros. This is based on the fact that the starting price is relatively high and that coins in similar conditions have reached hammer prices in that range.

Therefore, an estimated maximum purchase price could be approximately 600 euros.