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

1. Starting Price: 12,000 euros. This is the initial price of the auction.
2. Auction History: The coin has been auctioned 3 times in the past and has sold on all occasions, indicating a consistent interest in the coin.
3. Average Starting and Hammer Price: The average starting price is 14,666.67 euros and the average hammer price is 18,300 euros. This suggests that, in general, coins of this reference tend to sell for above the starting price.
4. Condition: The coin has a condition rating of 95 (S/C-), which is quite high and may positively influence its value.
5. Previous Prices: In previous auctions, the hammer prices have been 22,100 euros (2015) and 20,000 euros (2021), indicating that the value of the coin has been quite high in the past.

Given that the current starting price is 12,000 euros and considering the history of hammer prices, as well as the condition of the coin, an estimated maximum purchase price could be in the range of 18,000 to 22,000 euros. This is based on the fact that the most recent hammer price was 12,800 euros, but the interest and condition suggest it could reach a higher price.

Therefore, a reasonable estimated maximum purchase price could be 20,000 euros.