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

1. Condition: The coin has a condition rating of 95 (EF+), indicating that it is in very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction history:
- The average starting price in previous auctions is 21.3 euros, and the average hammer price is 13.5 euros.
- In past auctions, hammer prices have varied, but there are examples of sales reaching up to 24 euros (in a 2017 auction).

3. Sales trends: Of the 9 times it has been auctioned, it has sold 4 times, indicating moderate interest in the coin.

4. Current starting price: The starting price is 6 euros, which is relatively low compared to the average starting price of previous auctions.

Given that the coin is in good condition and considering previous hammer prices, an estimated maximum purchase price could be in the range of 20 to 30 euros. This is based on the fact that the highest hammer price in the past was 24 euros and the average hammer price is 13.5 euros, but given the condition, it can be expected that the hammer price will be higher in this auction.

Therefore, an estimated maximum purchase price could be 25 euros.