Based on the provided data, the coin in question has a grade of 85 (ebc-) and in a previous auction held on April 23, 2013, it had a starting price of 230.8 euros and a final price of 269.27 euros, with a grade of 90.

Considering that the average starting price is 150.4 euros and that the coin will be auctioned in the coming days, we can estimate a maximum purchase price based on the previous final price and the current grade. Since the current grade is slightly lower than the grade in the previous auction, we could consider a maximum purchase price slightly lower than the previous final price.

Therefore, my estimate for the maximum purchase price of this coin in the upcoming auction would be around 260-265 euros. It is important to note that this is just an estimate and the final price may vary depending on other factors in the auction.