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

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

2. Auction History: By looking at previous hammer prices, we can see that coins with higher conditions (70-80) have reached hammer prices ranging from 90 to 240 euros. However, the condition of this coin is significantly lower, suggesting that its hammer price will be lower.

3. Starting Price and Average Hammer Price: The starting price is 50 euros, and the average hammer price in the past has been 139.67 euros. However, given the low condition, it is likely that the actual hammer price will be below this average.

4. Auction Trends: The coin has been auctioned 18 times, of which 16 have sold. This indicates consistent interest, but the hammer price may vary depending on the condition.

Taking all these factors into account, an estimated maximum purchase price for this coin could be in the range of 50 to 80 euros, considering its low condition and the hammer price history of coins in similar conditions. A hammer price of around 70 euros could be a reasonable target, as it is a midpoint between the starting price and the average hammer price of coins in better condition.