Based on the provided data, we know that the coin in question has a conservation grade of 80 (MBC+), with a starting price in the upcoming auction of 30 euros. In the past, this coin has been auctioned 4 times, always sold, and its average auction price has been 77 euros.

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

1. In the auction on October 19, 2017, the coin had a starting price of 90 euros, an estimated price of 200 euros, and was auctioned for 190 euros, with a conservation grade of 70.
2. In the auction on December 13, 2017, the coin had a starting price of 12 euros, an estimated price of 20 euros, and was auctioned for 20 euros, with no information on conservation.
3. In the auction on September 14, 2022, the coin had a starting price of 9 euros, an estimated price of 15 euros, and was auctioned for 21 euros, with a conservation grade of 80.
4. In the auction on December 14, 2022, the coin has a starting price of 30 euros and an estimated price of 50 euros, with no information on the final auction price.

Given that the average auction price in the past has been 77 euros and considering the coin's good conservation grade (80 MBC+), we could estimate a maximum purchase price of around 80-90 euros in this upcoming auction. It is important to note that the final price may vary depending on the demand at the auction and other external factors.