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

1. Starting Price: 1800 euros.
2. Condition: The coin has a condition rating of 95 (S/C-), indicating that it is in very good shape.
3. Auction History:
- In December 2021, it was auctioned with an estimated price of 2500 euros and sold for 2200 euros.
- In September 2022, the starting price was 1500 euros, with an estimated price of 2500 euros and a final bid of 1700 euros.

4. Average Final Price: 1950 euros, which is a good indicator of market value.

Given that the coin has a history of final bids exceeding the current starting price and considering its good condition, a reasonable maximum purchase price could be in the range of previous final bid prices.

Taking all this into account, an estimated maximum purchase price could be approximately 2200 euros, which is the highest price reached in previous auctions for this condition. This would allow you to compete in the auction without exceeding the observed market value.