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

1. Current starting price: 350 euros.
2. Auction history: The coin has been auctioned 7 times and has sold on all occasions, indicating a constant interest in the coin.
3. Previous prices:
- In 2019, it sold for 440 euros with a grade of 95.
- In 2015, it sold for 420 euros, although the grade is not specified.
- In other auctions, the hammer prices were significantly lower, suggesting that the coin's grade influences the price.

4. Average hammer price: 273.43 euros, indicating that, in general, coins of this reference tend to sell below the current starting price.

5. Grade: There is no information about the grade of the coin to be auctioned, which introduces uncertainty. However, the starting price is considerably higher than the average hammer price.

Given that the starting price is 350 euros and considering that the average hammer price is 273.43 euros, an estimated maximum purchase price could be positioned between the average hammer price and the starting price, taking into account the possibility that the grade is good.

Estimated maximum purchase price: It could be reasonable to consider a maximum purchase price of around 400 euros, as it is slightly higher than the average hammer price and below the starting price, which could be attractive to a buyer looking for a good opportunity.