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 75 (MBC), indicating that it is in quite good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- It has been auctioned 4 times in the past, of which it has sold 2 times. This suggests that there is interest in the coin, although there is also a 50% no-sale rate, which may indicate that the starting price is sometimes too high.
- The average starting price in previous auctions is 57.5 euros, and the average hammer price is 72 euros. This provides a reference range for the coin's value.

3. Previous Prices:
- In the April 2020 auction, the coin started at 20 euros and sold for 72 euros, indicating that interest may be high if the starting price is attractive.
- In December 2019, the starting price was 60 euros, but it did not sell, which may indicate that the starting price was too high for market interest at that time.
- In October 2018, the starting price was 120 euros, but no sale was recorded, suggesting that the price was excessive.

4. Current Starting Price: The current starting price is 30 euros, which is significantly lower than the average starting price of previous auctions. This could attract more buyers.

Based on these factors, an estimated maximum purchase price could be in a range between the previous average hammer price (72 euros) and a bit more, given that the condition is good and the starting price is attractive.

Therefore, an estimated maximum purchase price could be around 80-90 euros. This would allow a competitive buyer in the auction, considering the good condition and historical interest in the coin.