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

1. Current Condition: The coin has a condition rating of 25 (BC), indicating that it is in quite low condition. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- The coin has been auctioned 6 times, of which it has sold 5 times. This suggests that there is interest in the coin, although its current condition may limit the price.
- Previous hammer prices have varied significantly, ranging from 20 to 500 euros, depending on the condition of the coin at each auction.

3. Average Hammer Price: The average hammer price for similar coins is 164 euros, but this includes coins in better condition.

4. Starting and Hammer Prices:
- The current starting price is 20 euros, which is relatively low.
- In previous auctions, hammer prices have been significantly higher, but they have also depended on the condition.

5. Estimation Based on Condition: Given that the condition of this coin is 25 (BC), it is reasonable to assume that its hammer price will be below the average of 164 euros. However, considering that there have been previous auctions with higher hammer prices, we could estimate a maximum purchase price.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 30 and 50 euros. This reflects the low condition, but also the historical interest in the coin.