Based on the data provided, we can observe that the coin in question has had an average auction closing price of 263.93 euros in previous auctions. Additionally, its current condition is 90 (EBC).

Considering that the coin will be auctioned in the upcoming days with a starting price of 400 euros, we can estimate a maximum purchase price based on the historical average closing price and the current condition. Given the high condition (90), it is likely that the closing price in the next auction will either match or exceed the historical average price.

Therefore, an estimated maximum purchase price for this coin in the next auction could be around 280-320 euros, considering the condition and previous closing prices. It is important to note that higher prices have been achieved in past auctions, so this range is a conservative estimate.