Based on the provided data, we can see that the coin in question has a conservation grade of 70 (MBC-), with a starting price in the upcoming auction of 18 euros. Additionally, it has been auctioned four times in the past, with an average starting price of 26 euros and an average closing price of 58.75 euros.

We also have information on previous auctions of this same coin:

1. In the auction on October 29, 2015, the coin had a starting price of 45 euros, an estimated price of 70 euros, and closed at 75 euros.
2. In the auction on May 28, 2014, the coin had a starting price of 15 euros, an estimated price of 25 euros, and closed at 15 euros.
3. In the auction on June 3, 2021, the coin had an estimated price of 9 euros and closed at 15 euros.

Given that the average closing price in previous auctions has been 58.75 euros and the coin has sold in all previous auctions, we could estimate a maximum purchase price around that value. However, considering that the conservation grade of the coin is 70 (MBC-), which is a medium-low grade, and the starting price is 18 euros, we could consider a maximum estimated purchase price of around 50-60 euros.