To estimate a maximum purchase price for the coin in question, we will consider several factors:

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

2. Auction history:
- The average hammer price in previous auctions is 18 euros.
- In the most recent auction (2020-12-15), the coin sold for 20 euros, which is a good indicator of its current value.
- In other auctions, hammer prices have varied, but many have been in the range of 20 to 25 euros.

3. Starting price and demand: The current starting price is 15 euros, which may attract more bidders. However, the starting price does not always reflect the final hammer price.

4. Price trends: Looking at previous hammer prices, it seems there is a trend for coins in good condition (like this one) to sell for over 20 euros.

Based on these factors, an estimated maximum purchase price could be between 25 and 30 euros, considering that the condition is high and there is demonstrated interest in previous auctions. This also allows for some margin for competition in the auction.