Based on the data provided, we can see that the coin in question has had an average auction closing price of 45 euros in previous auctions. Additionally, the condition of this coin has been described as '80 (VF+)'.

Considering that the coin will be auctioned in the upcoming days with a starting price of 12 euros, we can estimate a maximum purchase price based on the historical average closing price and the condition of the coin. Given that the condition is good but not excellent, and the average closing price has been 45 euros, we could consider a maximum purchase price around 50-55 euros.

It is important to note that the final auction price may vary depending on the demand at that specific moment, so it is advisable to monitor the auction's progress to adjust the offer accordingly.