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

1. Condition: The coin is in a condition of 100 (S/C), indicating that it is in excellent shape. This generally increases its value compared to coins in lower conditions.

2. Auction History:
- In the December 2021 auction, the coin was estimated at 5000 euros and sold for 11000 euros, showing significant interest and an increase in the final price.
- In March 2023, the coin had a starting price of 9000 euros and a final price of 9000 euros, indicating that it did not sell above its starting price, despite having an estimated price of 15000 euros.

3. Starting Prices and Average Prices:
- The current starting price is 8000 euros, which is lower than the average starting price of 8500 euros and the average final price of 10000 euros. This suggests that the starting price is attractive and could draw in more bidders.

4. Price Trends: Given that the coin has had a final price of up to 11000 euros in the past and considering its high condition, it is reasonable to expect that interest in the coin will remain.

Based on these factors, an estimated maximum purchase price could be in the range of 11000 to 13000 euros, considering that bidder interest could drive the price to a higher level, especially given the auction history and the coin's condition.

Therefore, a reasonable maximum purchase price could be 12000 euros.