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

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for coins of similar condition (75) in the past has been approximately 25 to 50 euros, with some cases reaching up to 90 euros.
- The last auction of a coin with a condition of 75 had a hammer price of 26 euros.

3. Starting Price and Average Hammer Price:
- The current starting price is 20 euros, and the average hammer price in the past has been 37.25 euros.

4. Price Trends:
- In previous auctions, hammer prices have shown a tendency to increase over time, especially for coins in good condition.

Based on this data, an estimated maximum purchase price for this coin could be in the range of 50 to 70 euros, considering that the condition is good and there have been previous auctions that reached higher prices. However, since the starting price is 20 euros, it is likely that the hammer price will not significantly exceed the mentioned range.

Therefore, a reasonable maximum purchase price could be 60 euros.