Based on the provided data, we can see that the coin was previously auctioned with a starting price of 12 euros, an estimated price of 20 euros, and a final price of 31 euros. The condition of the coin on that occasion was 30 (BC).

Given that the average auction price of this coin has been 25.5 euros and considering that the condition of the coin on this occasion is 25 (BC), we could estimate a maximum purchase price around 28-30 euros.

It is important to note that the final auction price may vary depending on factors such as demand at that specific moment, competition from other bidders, and interest in the coin in question.