Based on the data provided, we can observe that the coin in question has had varied starting and estimated prices in previous auctions. The current condition of the coin is 25 (BC), indicating that it is not in the best condition.

The starting price for the upcoming auction is 18 euros, and the average selling price in previous auctions has been 93.75 euros. Additionally, the highest estimated price in previous auctions was 500 euros.

Taking these data into consideration, as well as the history of previous prices, we could estimate a maximum purchase price of around 150-200 euros for this coin in the upcoming auction. Given that the condition is not the best, it is important to take this factor into account when determining the maximum purchase price.