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

1. Current Condition: The coin has a condition of 70 (VF-), indicating that it is in relatively good shape, but not among the best qualities.

2. Previous Prices:
- In the 2020 auction, the coin sold for 26 euros with a condition of 80.
- In 2017 and 2015, although the condition is not specified, the hammer prices were 90 and 75 euros respectively, with starting prices of 75 euros.
- In 2019, it sold for 18 euros with a starting price of 1 euro and an unspecified condition.
- In 2021, it sold for 15 euros with an estimated price of 9 euros and a condition of 25.

3. Starting Price and Average Price: The current starting price is 12 euros, and the average starting price in the past has been 35.6 euros, with an average hammer price of 41.5 euros.

4. Sales Trends: Since the coin has sold every time it has been auctioned, this indicates a consistent interest in it.

Based on this data, an estimated maximum purchase price could be positioned between the average hammer price (41.5 euros) and the highest observed hammer price (90 euros). However, since the current condition is lower than that of some previous auctions, a more conservative price might be more appropriate.

Therefore, a reasonable estimated maximum purchase price could be around 40-50 euros. This takes into account the current condition and the historical interest in the coin.