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 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 has been 68.71 euros.
- In previous auctions, hammer prices have varied significantly, but some relevant examples are:
- In 2022, a coin with a condition of 90 was sold for 200 euros.
- In 2020, a similar coin sold for 20 euros.
- In 2023, the hammer price was 55 euros.

3. Starting Price and Estimates:
- The current starting price is 15 euros, and the average starting price in the past has been 26.05 euros.
- The estimated price for this auction is 25 euros.

4. Sales Trends: Of the 20 times it has been auctioned in the past, it has sold 8 times, indicating a sales rate of 40%. This suggests that there is interest in the coin, but also that there is competition.

Based on these factors, an estimated maximum purchase price could be in the range of 70 to 100 euros, considering that the condition is good and there are examples of previous sales that reached higher prices. However, since the average hammer price is 68.71 euros, a reasonable maximum purchase price could be approximately 80 euros. This allows for a margin for competition in the auction and the possibility of the price rising due to demand.