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 current auction of 18 euros. It has been auctioned 3 times in the past, selling on all those occasions. The average starting price in previous auctions was 15 euros, while the average final price was 28.67 euros.

Additionally, we have information on two previous auctions of this same coin:

1. In the auction on July 2, 2020, the coin had a starting price of 12 euros, an estimated price of 20 euros, and sold for 21 euros. It had a conservation grade of 70.
2. In the auction on June 3, 2021, the starting price is not mentioned, but the estimated price was 9 euros, and it sold for 20 euros. It had a conservation grade of 30.

Given that the coin has sold in all previous auctions and its final price has consistently been higher than the starting price, we could expect the final price in this instance to be higher than the current starting price of 18 euros.

Considering the upward trend in final prices and the conservation grade of the coin, we could estimate a maximum purchase price around 30-35 euros. It is important to note that the final price will depend on the demand in the current auction and other factors that may influence buyers' decisions.