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

1. Starting Price: 200 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 70 (VF-), indicating that it is in fairly good shape, although not the best possible condition.
3. Auction History: The coin has been auctioned once in the past, with a starting price of 400 euros and a hammer price of 850 euros. This suggests that there is significant interest in this coin, and the hammer price was considerably higher than the starting price.
4. Average Starting and Hammer Prices: The average starting price is 300 euros, and the average hammer price is 850 euros. This reinforces the idea that the coin has good selling potential.

Given that the coin has a history of successful sales and a hammer price that significantly exceeds the starting price, and considering its condition, an estimated maximum purchase price could be positioned in a range between the average hammer price and the previous hammer price.

Therefore, an estimated maximum purchase price could be around 800 to 900 euros. This is based on the demonstrated interest in previous auctions and the condition of the coin.