To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data. Here are some key points to consider:

1. Starting price: 300 euros.
2. Average hammer price: 317.16 euros.
3. Hammer price history: Previous hammer prices for similar coins have varied, with some reaching up to 550 euros in recent auctions.
4. Condition: There is no information about the condition of this coin, which may affect its value. However, most coins in the history have conditions between 80 and 100, suggesting that the coin could be in a similar range.

Given that the average hammer price is 317.16 euros and considering that some similar coins have reached higher hammer prices, an estimated maximum purchase price could be in the range of 400 to 450 euros. This is based on the fact that the hammer price can be higher than the average price, especially if the coin is in good condition or if there is significant interest in the auction.

Therefore, an estimated maximum purchase price could be 450 euros.