Based on the data provided, we can observe that the coin in question has had an average auction closing price of 72.89 euros in previous auctions. Additionally, its current condition is graded as 30 (BC+).

Considering that the starting price for the upcoming auction is 15 euros and the historical average starting price is 37.88 euros, we can estimate that the closing price could be around 72.89 euros, which is the historical average closing price.

Given that the coin's condition is 30 (BC+), which is acceptable but not excellent, and considering that the coin has been sold multiple times in the past, we could expect the closing price to remain within a similar range to the historical average price.

Therefore, my estimate for the maximum purchase price for this coin in the upcoming auction would be around 70-75 euros. It is important to note that this is just an estimation based on the provided data and the history of previous auctions.