To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the starting and hammer prices from previous auctions, as well as the condition of the coin.

1. Current starting price: 400 euros.
2. Average hammer price in the past: 305.67 euros.
3. Previous hammer prices:
- The most recent auction shows a hammer price of 550 euros with a starting price of 400 euros.
- Other recent hammer prices have ranged from 300 to 360 euros.

4. Condition: No information is provided about the condition of this coin, which may complicate the estimation. However, by observing the hammer prices of coins with similar conditions (around 80-95), we can infer that the price could be in a higher range if the condition is good.

5. Price trends: In previous auctions, hammer prices have varied, but there has been an increase in hammer prices in recent years, suggesting an upward trend.

### Estimation of the maximum purchase price:
Given that the starting price is 400 euros and considering that recent auctions have reached hammer prices of up to 550 euros, a reasonable maximum purchase price could be in the range of 500 to 600 euros, assuming the coin has an acceptable condition and that interest in it remains.

Estimated maximum purchase price: 550 euros.