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 70 (MS-). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for coins of similar grade (70) in the past has been approximately 52.02 euros.
- In previous auctions, coins with a grade of 70 have reached hammer prices of up to 65 euros (as seen in the auction on September 19, 2019).

3. Starting Price and Average Starting Price:
- The current starting price is 12 euros, which is quite low compared to the average starting price of 31.39 euros.
- This suggests that there is potential for the hammer price to be significantly higher.

4. Recent Trends:
- In the most recent auctions, hammer prices for coins of similar grade have varied, but they have shown a range that can reach up to 60-70 euros in some cases.

5. Data-Based Estimation:
- Given that the average hammer price for coins of grade 70 is 52.02 euros and considering that there is a history of hammer prices reaching up to 65 euros, an estimated maximum purchase price could be in the range of 60 to 70 euros.

Conclusion: Based on the analysis of the data, an estimated maximum purchase price for this coin could be 65 euros. This is based on the history of hammer prices and the current grade of the coin.