To estimate a maximum purchase price for the coin in question, we can consider several factors:

1. Current Condition: The coin has a condition rating of 95 (S/C-), indicating that it is in very good shape. This generally increases its value compared to coins in lower condition.

2. Auction History:
- The average hammer price in previous auctions is 431.06 euros.
- In past auctions, the coin has reached hammer prices of up to 777 euros (in 2015) and had a hammer price of 440 euros in recent auctions.
- The trend of hammer prices seems to be in the range of 400 to 500 euros in the most recent auctions.

3. Starting Price and Estimates:
- The current starting price is 400 euros, which is consistent with previous starting prices.
- The estimated price for this auction is 500 euros, suggesting that bidders are willing to pay more than the starting price.

4. Demand and Supply: The coin has been auctioned 25 times, with a 68% sell-through rate (17 out of 25). This indicates considerable interest in the coin.

Based on these factors, an estimated maximum purchase price could be around 500-600 euros, given that the estimated auction price is 500 euros and considering that it has reached higher prices in the past. However, to be conservative and considering the recent trend, a reasonable maximum purchase price could be approximately 550 euros.