Based on the provided data, the coin has a starting price of 12 euros and has been auctioned once in the past, selling for an average closing price of 30 euros. The coin's condition is graded as 30 (BC+). Since there are no previous prices provided for this coin, we can rely on the average closing price to make an estimation.

Considering that the coin sold for 30 euros in the previous auction and taking into account the condition being 30 (BC+), we could estimate a maximum purchase price around 35-40 euros, considering that the condition is not the best but also not the worst.

It is important to note that the final auction price may vary depending on the demand from buyers at that specific time, so it is advisable to monitor the auction to adjust the offer accordingly.