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

1. Current Condition: The coin has a grade of 70 (MS-), indicating that it is in relatively good condition. This suggests that its value should be above that of coins in worse conditions.

2. Previous Prices: By looking at the hammer prices from previous auctions, we can see that coins with similar grades (around 70) have had varying hammer prices. For example:
- In 2019, a coin with a grade of 70 was sold for 25 euros.
- In 2020, another coin with a grade of 75 was sold for 55 euros.
- In 2021, a coin with a grade of 75 was sold for 70 euros.

3. Average Hammer Price: The average hammer price for this coin is 83.37 euros, indicating that, in general, coins of this reference tend to reach higher prices.

4. Starting Price and Estimates: The current starting price is 18 euros, and the average starting price is 45.64 euros. This suggests that the current starting price is relatively low compared to the historical average.

5. Auction Trends: Given that there have been 22 previous auctions and 20 sales, demand seems to be good, which may influence a higher hammer price.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 70 to 90 euros, considering that the condition is good and the historical average hammer price is 83.37 euros. Therefore, a reasonable maximum purchase price could be 85 euros.