To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Current starting price: 900 euros.
2. Average starting price in the past: 1200 euros.
3. Condition of the coin: 95 (S/C-), indicating good condition.
4. Auction history: The coin has been auctioned once in the past but did not sell, which may indicate that interest or demand was insufficient at that time.
5. Previous starting price: 1500 euros, although it did not sell.

Given that the condition is high (95) and the average starting price is 1200 euros, we could consider the current starting price of 900 euros to be attractive. However, the fact that it did not sell at the previous auction for 1500 euros suggests that there may be a limit to demand.

Taking all these factors into account, an estimated maximum purchase price could be positioned between the current starting price and the average starting price. A reasonable range could be:

Estimated maximum purchase price: 1100 - 1200 euros.

This reflects an interest in the coin, considering its good condition and the average starting price, but also takes into account the lack of sales in previous auctions.