Based on the data provided, we can observe that the coin in question has had an average auction closing price of 284.18 euros in previous auctions. Additionally, its current condition is 90 (EBC).

Considering that the starting price for the next auction is 400 euros and that the coin has had an average auction closing price of 284.18 euros in the past, we could estimate a maximum purchase price around 350-380 euros, taking into account the upward trend in closing prices and the condition of the coin.

Therefore, my estimate for the maximum purchase price for this coin in the next auction would be around 350-380 euros.