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

1. Condition: The coin has a condition rating of 70 (VF-), indicating that it is in relatively good shape. This suggests that its value should be above that of coins in worse conditions.

2. Previous auction prices:
- In 2018, the coin was auctioned with a starting price of 75 euros and sold for 85 euros.
- In 2015, the starting price was 138.68 euros, although the final price is not indicated.
- In 2019 and 2020, the starting prices were 65 and 50 euros, respectively, but the outcomes of those auctions are unknown.

3. Average starting and final prices: The average starting price is 73.74 euros, and the average final price is 85 euros. This indicates that, in general, coins of this reference tend to sell within a similar range.

4. Current auction situation: The current starting price is 40 euros, which is significantly lower than previous starting prices. This could attract more bidders, potentially increasing the final price.

Given that the condition is good and considering previous final prices, an estimated maximum purchase price could be in a range between the average final price (85 euros) and the highest recorded final price (85 euros in 2018).

Therefore, an estimated maximum purchase price could be approximately 85-90 euros. This takes into account the possibility that the auction may attract interest due to the low starting price and the good condition of the coin.