Based on the provided data, the coin in question has a conservation grade of 85 (ebc-) and in its only past auction had a starting price of 18 euros and an estimated price of 30 euros, with a conservation value of 80. Since the coin will be auctioned soon with a starting price of 40 euros, we can consider that its value has increased since the last auction.

Considering that the coin is in fairly good conservation condition (85 ebc-), and that its estimated price in the last auction was 30 euros, we could expect the final auction price to be higher. Given that the starting price is 40 euros, and taking into account the upward trend in price, we could estimate a maximum purchase price around 45-50 euros. However, keep in mind that the final price will depend on the demand at the auction and other factors that may influence the bidding.