Based on the data provided, we can observe that the coin in question has had an average auction closing price of 153.28 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 225 euros and has had a maximum estimated purchase price of 850 euros in the past, we can estimate a maximum purchase price for this coin in the next auction.

Given that the average auction closing price has been 153.28 euros and the maximum estimated purchase price in the past was 850 euros, we could consider an estimated maximum purchase price for this coin in the next auction around 800-900 euros, taking into account its condition and historical demand.

Therefore, my estimation of the maximum purchase price for this coin in the next auction would be around 800-900 euros.