To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Starting Price: 15,000 euros. This is the initial price expected to be obtained at the auction.
2. Condition: The coin has a condition rating of 95 (S/C-), indicating that it is in very good shape. This generally increases its value.
3. Auction History:
- It has been auctioned 3 times and has sold on all occasions.
- In the last auction (2021), it was estimated at a price of 6,000 euros and sold for 19,000 euros, indicating great interest and potential for appreciation.
4. Previous Prices: Previous hammer prices are significantly lower than the current starting price, but the last hammer shows a substantial increase in value.

Given that the coin has a high condition rating and has shown increasing interest in previous auctions, the hammer price could be considerably higher than the starting price. However, it is also important to consider the average hammer price of 6,447.67 euros in previous auctions.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the starting price and the highest previous hammer price.

Therefore, a reasonable maximum purchase price could be around 18,000 to 20,000 euros, given the demonstrated interest in previous auctions and the high condition of the coin.