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

1. Current starting price: 1200 euros.
2. Condition: The coin is in a condition of 100 (S/C), indicating that it is in excellent shape.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 1800 euros and sold for 3200 euros.
- In 2021, although there was no starting price, it was estimated at 2000 euros and sold for 1500 euros.

4. Average starting and final prices:
- Average starting price: 1500 euros.
- Average final price: 2350 euros.

Given that the coin is in high condition and has a history of successful sales, the final price is usually significantly higher than the starting price.

Considering the current starting price of 1200 euros and the history of final prices, an estimated maximum purchase price could be positioned between the average final price (2350 euros) and the highest recorded final price (3200 euros).

Therefore, a reasonable estimated maximum purchase price could be around 2500 to 3000 euros, considering that the starting price is low compared to the final price history and the high condition of the coin.