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

1. Current Condition: The coin has a condition of 85 (ebc-), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices for coins with similar conditions (around 80-85), we can see that auction prices have varied, but generally, they have exceeded the starting price on many occasions. For example:
- In 2020, a coin with a condition of 85 was auctioned for 380 euros with a starting price of 250 euros.
- In 2021, a coin with a condition of 80 was auctioned for 700 euros.

3. Average Auction Price: The average auction price for this coin is 498.94 euros, suggesting that, in general, buyers are willing to pay close to this price.

4. Starting Price: The starting price is 400 euros, which is a good starting point, but since the average auction price is significantly higher, this suggests there is potential for a higher auction price.

5. Auction Trends: The coin has been auctioned 19 times, with a sell-through rate of 94.74% (18 out of 19), indicating consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 700 euros, considering that the condition is good and that previous auction prices have exceeded 600 euros on occasions. Therefore, a reasonable maximum purchase price could be 650 euros.