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

1. Current Condition: The coin has a condition of 70 (VF-), indicating that it is in an acceptable state, but not in the best condition.

2. Auction History:
- The coin has been auctioned 13 times, of which it has sold 12 times.
- The average hammer price in previous auctions is 27.18 euros.
- The hammer prices in previous auctions for coins with a similar condition (70) have varied, but the last hammer price was 40 euros.

3. Starting and Estimated Prices:
- The current starting price is 20 euros, and the average starting price in the past has been 18.5 euros.
- The estimated prices in previous auctions for coins with a condition of 70 have been up to 30 euros.

4. Trends:
- In the most recent auctions, the hammer price has shown a trend of increasing, reaching up to 40 euros in the last auction.

Based on this data, an estimated maximum purchase price could be set in a range between 30 and 40 euros, considering that the condition is 70 and that the highest hammer price in the past was 40 euros.

Therefore, a reasonable maximum purchase price could be 35 euros. This allows for a margin for competition in the auction and aligns with previous hammer prices.