Based on the data provided, we can observe that the coin in question has had an average starting price in past auctions of 17.6 euros and an average closing price of 20.71 euros. Additionally, its condition has been maintained at a level of 80 (MBC+).

Taking into account the previous prices of this coin, as well as its condition and the fact that it has been auctioned 26 times in the past, we can estimate a maximum purchase price for this coin in the upcoming auction.

Given that the average closing price has been 20.71 euros and the coin's condition remains high, we could consider an estimated maximum purchase price around 25-30 euros. This is based on the trend of previous prices and the demand this coin has shown in past auctions.

It is important to note that the final auction price may vary depending on factors such as competition from other bidders and the specific interest in this coin in the particular auction.