Based on the data provided, we can see that the coin in question has had an average auction closing price of 20.71 euros in previous auctions. Additionally, the coin's condition has been maintained at a level of 75 (MBC) in past auctions.

Given that the starting price for the upcoming auction is 12 euros and the historical average closing price is 20.71 euros, we could estimate a maximum purchase price around that range. However, considering that the coin's condition is 75 (MBC), we could adjust that price slightly downwards.

Therefore, my estimate for the maximum purchase price of this coin in the upcoming auction would be around 18-20 euros. This takes into account the condition and historical closing prices.