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 fairly low condition. This will negatively affect its value compared to coins in better condition.

2. Previous Auction Prices: By looking at the hammer prices from previous auctions, we can see that prices have varied considerably, but generally, coins in better condition (such as those rated 80) have reached hammer prices of up to 280 euros.

3. Average Hammer Price: The average hammer price for this coin is 186.9 euros, which gives us a reference for what buyers are willing to pay on average.

4. Starting Price: The current starting price is 70 euros, suggesting that the seller has moderate expectations.

5. Auction Trends: Since the coin has been auctioned 10 times and all have sold, this indicates a consistent interest in the coin, although the current condition may limit the price.

Taking all these factors into account, and given that the condition is quite low, an estimated maximum purchase price could be in the range of 100 to 150 euros. This is based on the fact that, although the coin has a history of higher prices, its current state limits its appeal to collectors.

Therefore, an estimated maximum purchase price could be 120 euros.