Based on the provided data, we have that the coin in question has a conservation grade of 85 (ebc-), with previous auction prices in 2014 and 2015 of 12.1 and 6.9 euros respectively, both with a conservation grade of 75. The starting price for the upcoming auction is 25 euros, with a historical average auction price of 28.45 euros.

Given that the coin has been auctioned in the past at prices lower than the current starting price, but has had a higher average auction price, we could consider a maximum estimated purchase price based on the historical average auction price.

Taking these factors into consideration, a maximum estimated purchase price for this coin could be around 30 euros. This takes into account the conservation grade of the coin and the previous auction prices.