Based on the data provided, we can see that the coin in question has had an average auction closing price of 45 euros in previous auctions. Additionally, the condition of this coin is graded at 90 (EBC).

Considering that the starting price in the upcoming auction is 18 euros and that the coin has performed well in previous auctions, we could estimate a maximum purchase price around 50-55 euros. This is because the average closing price has consistently been higher than the starting price, and the condition of the coin is quite good.

Therefore, an estimated maximum purchase price for this coin in the next auction could be around 50-55 euros.