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

1. Condition: The coin has a condition of 70 (MS-), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction history:
- The average hammer price for coins of similar condition (70) has been approximately 19.56 euros.
- In previous auctions, hammer prices for coins with a condition of 70 have ranged between 10 and 30 euros, with a most recent hammer price of 15 euros.

3. Starting price and previous prices:
- The current starting price is 12 euros, which is slightly lower than the average starting price of 13.82 euros.
- The estimated price for this auction is 20 euros, suggesting that there are expectations for the coin to sell at a higher price.

4. Sales trends: Of the 27 times it has been auctioned, it has sold 19 times, indicating good demand and consistent interest in this coin.

Taking all these factors into account, an estimated maximum purchase price could be around 25-30 euros, as this range aligns with previous hammer prices and the condition of the coin. However, it is important to consider that the final price may vary depending on competition in the auction and the interest of other bidders.