To estimate a maximum purchase price for the coin that is going to be auctioned, we can consider several factors based on the provided data:

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

2. Auction history:
- The average hammer price for this coin in the past has been 22.22 euros.
- In previous auctions, the hammer price has reached up to 80 euros on one occasion (auction on 2019-01-23).
- Most auctions have had hammer prices ranging between 15 and 45 euros.

3. Starting price and estimates:
- The current starting price is 20 euros, which is slightly higher than the average starting price of 15.24 euros.
- The estimated price for this auction is 30 euros, suggesting that the coin is expected to reach a value above the starting price.

4. Sales trends: Of the 24 times it has been auctioned, it has sold 18 times, indicating good demand and consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 45 to 80 euros, given that the condition is high and there are precedents for sales in that range. However, to be more conservative and considering the starting price and the average hammer price, a reasonable maximum purchase price could be approximately 60 euros. This allows for competition in the auction and the possibility that other bidders may also be interested in the coin.