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

1. Current Grade: The coin has a grade of 75 (MS), indicating that it is in fairly good condition, but not the best possible grade.

2. Previous Prices: By looking at previous auction prices for coins with similar grades (75-80), we can see that auction prices have varied. For example:
- In 2015, a coin with a grade of 75 sold for 16 and 69.84 euros.
- In 2020, a coin with a grade of 85 sold for 70 euros.
- In 2022, a coin with a grade of 85 sold for 125 euros.

3. Average Auction Price: The average auction price for this coin is 52.88 euros, which can serve as a good indicator of the expected value.

4. Starting Price: The starting price is 20 euros, suggesting that the auctioneer expects moderate interest in the coin.

5. Auction Trends: The coin has been auctioned 45 times, of which it has sold 27 times. This indicates a consistent interest in the coin, which may increase its value.

Based on these factors, an estimated maximum purchase price could be in the range of 60 to 80 euros, considering that the grade is good and there is a history of auction prices supporting this estimate.

Therefore, a reasonable maximum purchase price could be 70 euros.