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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Auction History:
- The average hammer price for similar coins in the past has been 25.71 euros.
- The last auction of a coin with a similar condition (95) had a hammer price of 50 euros, suggesting that coins in good condition can reach higher prices.

3. Price Trends:
- In previous auctions, hammer prices have varied, but there are examples of coins with conditions ranging from 80 to 95 that have sold between 20 and 90 euros.
- The coin in question has a starting price of 15 euros, which is relatively low compared to previous hammer prices.

4. Demand and Supply: Since it has been auctioned 57 times and sold 51 times, there is a steady demand for this coin, which may drive up its price in the next auction.

Based on these factors, an estimated maximum purchase price for the coin in question could be between 40 and 60 euros, considering its good condition and the history of hammer prices. This allows for some margin for competition in the auction and the possibility that other bidders may be interested in the coin.