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

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

2. Auction History:
- The average hammer price for similar coins in the past is 17.27 euros.
- The coin has been auctioned 50 times, of which it has sold 22 times, indicating a sell-through rate of 44%. This suggests that there is interest in the coin, but also that there is competition.

3. Previous Prices:
- In previous auctions, hammer prices have varied considerably, but there are examples of hammer prices reaching up to 225 euros (in the case of a condition of 85).
- The last auction of a coin with a condition of 100 had a hammer price of 40 euros.

4. Starting Price and Estimates:
- The starting price is 20 euros, which is a good starting point, but given the condition, it is likely that the hammer price will be significantly higher.

Taking all these factors into account, an estimated maximum purchase price could be between 40 and 60 euros, considering that the condition is high and there is demonstrated interest in previous auctions. However, given the price history, a maximum of 60 euros seems reasonable, as it aligns with the hammer prices of coins in similar conditions.