Based on the provided data, we can see that the coin in question has a conservation grade of 30 (BC+), indicating that it is in an acceptable but not excellent condition.

Furthermore, it has been auctioned three times in the past with starting prices of 75, 35, and 15 euros respectively. The estimated prices in those auctions were 125, 50, and 30 euros, and the final prices were 85 and 55 euros.

The current starting price is 12 euros, with a historical average starting price of 34.25 euros and a historical average final price of 56.67 euros.

Considering the coin's acceptable condition and previous final prices of 85 and 55 euros, and taking into account the current starting price of 12 euros, we could estimate a maximum purchase price around 70-80 euros, based on conservation and previous final prices.

It's important to note that the final auction price will depend on the demand at that time and other factors that may influence potential buyers' decisions.