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 (MS+/EF-), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current Listing Price and Previous Prices: The current listing price is 500 euros. In previous auctions, hammer prices for coins with a similar condition (85) have varied, but many have reached or exceeded the current listing price. For example, in past auctions, the hammer price for coins with a condition of 85 has ranged from 320 to 750 euros.

3. Price Trends: The average hammer price for similar coins is 392.19 euros, suggesting that the market is willing to pay close to this value. However, since the coin in question has a higher listing price, bidders may be willing to pay more.

4. Sales History: The coin has sold in 64 out of the last 68 auctions, indicating good demand. This suggests that there is consistent interest in this coin, which may push the price up.

5. Proximity of the Auction: Since it will be auctioned in the coming days, anticipation may increase interest and, therefore, the price.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 600 to 750 euros. This range considers the good condition of the coin, the history of hammer prices, and market demand.