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

1. Current Condition: The coin has a condition rating of 20 (BC-), which indicates that it is in quite poor condition. This generally reduces the value of the coin compared to coins in better condition.

2. Previous Prices: By looking at previous auction prices, we can see that in past auctions, coins with higher condition ratings have reached significantly higher prices. For example, in 2023, a coin with a condition rating of 90 was auctioned for 320 euros, while one with a condition rating of 70 sold for 211 euros.

3. Average Auction Price: The average auction price for this coin is 229.46 euros, which can serve as a reference point.

4. Starting Price: The current starting price is 150 euros, indicating that the seller expects a minimum of 150 euros.

5. Sales Trends: The coin has been auctioned 40 times, of which 36 have sold. This suggests that there is consistent interest in the coin, although the low condition may limit the number of buyers willing to pay high prices.

Given that the condition of the coin is low (20), and considering the auction prices of coins in similar conditions, an estimated maximum purchase price could be in the range of 100 to 150 euros. However, since the starting price is 150 euros, it is likely that the final auction price will not significantly exceed this value.

Therefore, an estimated maximum purchase price could be 150 euros.