Based on the data provided, we can see that the coin in question has had a starting price in the current auction of 15 euros, with a conservation value of 20 (BC-). Additionally, it has been auctioned in the past on several occasions, with closing prices ranging between 17 and 570 euros.

The average starting price of previous auctions has been 71.88 euros, while the average closing price has been 123.24 euros. This indicates that previous auctions have seen a significant increase in the final price.

Given that the coin is in a conservation state of 20 (BC-), and considering the previous closing prices as well as the increase in the average closing price, we could estimate a maximum purchase price of around 150 euros for this coin in the current auction. It is important to note that this is just an estimate and the final price may vary depending on demand at the auction.