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

1. Current Condition: The coin has a condition rating of 70 (VF-). This indicates that it is in an acceptable state, but not of high quality. In comparison with previously auctioned coins, a condition of 70 is similar to one from past auctions.

2. Previous Prices:
- In the 2020 auction, a coin with a condition of 75 had a starting price of 18 euros, but it did not sell.
- In 2022, a coin with a condition of 90 sold for 140 euros, indicating that the price can significantly increase with better condition.
- In another 2022 auction, a coin with a condition of 70 had a starting price of 15 euros, but it did not sell.

3. Average Hammer Price: The average hammer price for similar coins is 140 euros, suggesting that there is a market willing to pay high prices for coins in good condition.

4. Starting Price and Estimates: The current starting price is 12 euros, which is quite low compared to the average hammer price. The average starting price of 31.25 euros also suggests that there is potential for price increase.

5. Demand and Supply: Given that there is an auction scheduled in the coming days and considering that there have been more auctions without sales, this may influence demand.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 30 to 50 euros, considering that the condition is a limiting factor and that the hammer price may be higher depending on demand at the auction.

Therefore, a reasonable maximum purchase price could be 40 euros.