To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current condition of the coin.

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

2. Previous Auction Prices: By observing the auction prices of coins with a similar condition (70), we can see that they have varied. Some examples are:
- 2020-09-17: Auction price 25
- 2020-05-26: Auction price 18
- 2019-07-03: Auction price 18
- 2018-10-15: Auction price 10
- 2022-10-17: Auction price 21

3. Price Trends: The average auction price for this coin is 39.8 euros, but this includes coins in better conditions. Given that the condition of the coin at auction is 70, it is reasonable to expect that the auction price will be below this average.

4. Starting Price: The starting price is 12 euros, which suggests that the auctioneer expects moderate interest in the coin.

5. Estimation: Based on previous auction prices and the current condition, a reasonable maximum purchase price could be in the range of 25 to 35 euros. This aligns with previous auction prices for coins of similar condition and also takes into account the starting price.

Therefore, an estimated maximum purchase price for this coin could be 30 euros.