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

1. Current Condition: The coin has a condition rating of 70 (MBC-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- In the past, the coin has been auctioned 4 times, of which it has sold 3 times.
- Previous hammer prices for coins with a similar condition (70) have been 25 and 45 euros, indicating a price range in which it has moved.

3. Starting Price and Average Hammer Price:
- The current starting price is 20 euros, which is below the average starting price of 22.5 euros.
- The average hammer price is 33.33 euros, suggesting that buyers are willing to pay more than the starting price.

4. Price Trends:
- In the most recent auction, the coin sold for 25 euros, indicating that interest in this coin remains.

Based on these factors, an estimated maximum purchase price could be positioned in a range between the average hammer price and the highest observed hammer price. Given that the average hammer price is 33.33 euros and the highest hammer price was 45 euros, a reasonable maximum price could be:

Estimated maximum purchase price: 40 euros.

This price reflects a balance between the coin's condition, price history, and market interest.