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

1. Current starting price: 25 euros.
2. Coin condition: 30 (BC+), indicating an acceptable but not excellent condition.
3. Auction history:
- In 2020, it was sold for 45 euros with a condition of 30.
- In 2019, it was sold for 41 euros, although the condition is not specified.
- In 2014, it was sold for 42 euros, without specifying the condition.

4. Average hammer price: 42.67 euros, suggesting that, in general, coins of this reference tend to sell above the starting price.

Given that the coin has a history of sales exceeding the current starting price and considering that the condition is the same as in the 2020 auction, an estimated maximum purchase price could be around 45 euros, which is the highest price reached in the past for a coin with the same condition.

Therefore, an estimated maximum purchase price for this coin could be 45 euros.