Based on the provided data, the coin in question has a conservation grade of 30 (BC+), with a starting price in the upcoming auction of 18 euros. In the past, it has been auctioned once with a starting price of 12 euros and a closing price of 50 euros, with a conservation grade of 20.

Considering that the average starting price is 15 euros and the average closing price is 50 euros, and taking into account the history of previous auctions where the estimated price was 20 euros, we could estimate a maximum purchase price for this coin at around 25-30 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 pay attention to the auction to adjust the offer accordingly.