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

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 Starting Price and Previous Prices: The current starting price is 390 euros. In previous auctions, the average hammer price has been 493.19 euros, suggesting that buyers are willing to pay more than the starting price on many occasions.

3. Price Trends: By observing the hammer prices from previous auctions, we can see that they have varied, but overall, prices have been above the starting price. For example, in the auction on 2022-05-07, the coin sold for 460 euros with a starting price of 350 euros.

4. Comparison with Similar Condition Prices: In recent auctions, coins with a similar condition (85) have reached hammer prices ranging from 440 to 480 euros.

Based on these factors, an estimated maximum purchase price could be in the range of 480 to 500 euros. This is because, given the price history and the condition of the coin, it is reasonable to expect that buyers would be willing to pay a price in this range, especially if there is interest in the auction.

Therefore, an estimated maximum purchase price could be 500 euros.