Based on the information provided, we have a coin with a conservation grade of 90 (EBC) that will be auctioned soon with a starting price of 75 euros. Since there have been no previous sales of this coin, we do not have a historical average auction price for this specific reference.

Considering the conservation grade of the coin (90 EBC) and the starting price (75 euros), as well as the fact that this coin is being auctioned for the first time, we could estimate a maximum purchase price around 90-100 euros. It is important to note that the final auction price can vary depending on demand and other factors, so this estimate is just an approximate guideline.