Based on the data provided, we can observe that the coin in question has had an average auction closing price of 24.55 euros in previous auctions. Additionally, the current condition of the coin is 30 (BC+).

Considering that the current condition is 30 (BC+), and the average auction closing price in previous auctions has been 24.55 euros, we can estimate a maximum purchase price for this coin in the upcoming auction.

Given that the current condition is not the best (FDC), and the average auction closing price has been 24.55 euros, we could consider a maximum purchase price around 20-25 euros, taking into account that condition is an important factor in the value of a coin for collectors.

Therefore, my estimation of the maximum purchase price for this coin in the upcoming auction would be around 20-25 euros.