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

1. Condition: The coin has a condition rating of 20 (BC-), indicating that it is in quite poor condition. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- It has been auctioned 4 times in the past, of which it has sold 3 times.
- The average hammer price in previous auctions is 90 euros, giving us an idea of the value that buyers are willing to pay.

3. Previous Prices:
- In 2020, the coin was auctioned with a starting price of 12 euros and sold for 80 euros.
- In 2016, the starting price was 80 euros, and it sold for 80 euros.
- In 2022, the starting price was 200 euros, but it did not sell, indicating that the starting price was too high for the coin's condition.

4. Current Starting Price: The starting price is 110 euros, which is higher than previous starting prices, suggesting that the seller expects a higher value, but it could also be a risk if the condition does not justify that price.

Given that the condition is low and considering the auction price history, an estimated maximum purchase price could be around 80-90 euros, which is the range where it has sold in the past. However, since the starting price is 110 euros, it may be wise not to exceed 80 euros to avoid overpaying for a coin in this condition.

Therefore, an estimated maximum purchase price could be 80 euros.