Based on the data provided, we can observe that the coin in question has had an average auction closing price of 73.53 euros in previous auctions. Additionally, its current condition is estimated at 30 (BC+).

Considering that the starting price for the next auction is 15 euros and the historical average closing price is 73.53 euros, we can assume that there is a significant negotiation margin in the auctions of this coin.

Given the current condition of 30 (BC+), and taking into account the previous prices of the coin as well as its auction history, we could estimate a maximum purchase price around 60-80 euros. This range is based on the trend of previous prices and the possibility of obtaining a good closing price in the next auction, considering the condition and the interest that this coin has generated in previous auctions.