Given that the coin is in a condition of 90 (EBC - Excellent Good Condition) and has not been auctioned or sold in the past, the starting price of 500 euros seems reasonable. However, since there are no historical sales data for this reference, it is difficult to establish a precise estimated maximum purchase price.

In general, coins in good condition (EBC) can reach prices above the starting price, especially if there is interest in the auction. Considering that this is the first time it is being auctioned and that the starting price is 500 euros, an estimated maximum purchase price could be in the range of 700 to 900 euros, depending on the interest of collectors and competition in the auction.

Therefore, an estimated maximum purchase price could be approximately 800 euros.