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

1. Condition of the coin: The coin has a condition rating of 80 (MBC+), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current listing price and previous prices: The current listing price is 120 euros. In previous auctions, we have seen hammer prices that varied significantly, but some have reached prices above 100 euros, such as the hammer price of 103 euros in 2019.

3. Price trends: The average hammer price in previous auctions is 74.05 euros, but there are examples of sales that have exceeded 100 euros. This suggests that there is potential for the coin to sell at a higher price, especially considering its good condition.

4. Supply and demand: Since this coin has been auctioned 20 times and all have sold, this indicates good demand. Additionally, it will be auctioned once more in the coming days, which may influence competition and the final price.

Taking all these factors into account, an estimated maximum purchase price could be around 120-140 euros. This is based on the good condition of the coin and previous hammer prices that have exceeded 100 euros. However, it is important to consider that the final price will depend on the competition in the auction and the interest of collectors.