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

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

2. Auction History:
- The average hammer price in previous auctions is 13.89 euros.
- The coin has been auctioned 26 times, of which it has sold 18 times, indicating good demand.
- Previous hammer prices varied, but there are examples of auctions reaching up to 45 euros in similar conditions.

3. Starting Price and Estimates:
- The current starting price is 8 euros, and the average starting price in the past has been 8.69 euros.
- The estimated price for this auction is 15 euros, suggesting expectations of an increase in the hammer price.

4. Recent Trends: In the most recent auctions, hammer prices have shown an increase, which may indicate a positive trend in the coin's value.

Taking all these factors into account, an estimated maximum purchase price could be around 20-25 euros, considering that the condition is good and there is a history of hammer prices supporting this figure. However, it is important to be aware of the competition in the auction, as this may influence the final price.