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 20 (BC-), indicating that it is in quite low condition. This generally negatively affects the hammer price.

2. Auction History:
- The average hammer price in previous auctions is 43.8 euros.
- The coin has been auctioned 18 times, of which it has sold 15 times. This suggests that there is consistent interest in the coin, even though its current condition is low.
- In previous auctions, hammer prices have varied, but generally, hammer prices for higher conditions have been significantly greater.

3. Starting and Hammer Prices:
- The current starting price is 25 euros, which is relatively low compared to the average hammer price.
- In previous auctions, the highest hammer price for a similar condition was 50 euros (in 2022).

4. Price Trends:
- The price trend shows that, despite the low condition, there is a price range that has remained around 30-50 euros in recent auctions.

Given all this, an estimated maximum purchase price for this coin, considering its low condition and price history, could be around 30-35 euros. This would allow for a competitive bid without exceeding the value that collectors would be willing to pay for a coin in this condition.