To estimate a maximum purchase price for the coin in question, we can analyze several factors:

1. Condition: The coin has a condition rating of 80 (MS+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for similar coins is 485.67 euros.
- In previous auctions, hammer prices have varied, but there are examples of sales reaching up to 850 euros (as seen in the 2015 auction).
- The coin has been auctioned 13 times, of which 10 have been sold, indicating a consistent interest in this reference.

3. Starting Price: The starting price is 400 euros, which is a good starting point. However, the starting price does not always reflect the final hammer price.

4. Price Trends: In recent auctions, hammer prices for coins with a similar condition have been quite high, suggesting that there is active demand.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 750 euros, considering that the average hammer price is 485.67 euros and that there are examples of higher hammer prices in the past.

Therefore, a reasonable maximum purchase price could be 700 euros.