To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the conditions of the auctioned coins.

1. Current starting price: 500 euros.
2. Average hammer price: 225.78 euros.
3. Hammer price history:
- The coin has had hammer prices that vary significantly, ranging from 15 euros to 850 euros in previous auctions.
- The conditions of the auctioned coins in the past that have higher hammer prices (such as 850 euros) have a condition rating of 95, indicating they are of high quality.

4. Condition of the current coin: No information is provided about the condition of the coin to be auctioned, making the estimation more complicated. However, since the starting price is 500 euros, it is reasonable to assume that the coin has a condition rating that could be in the medium-high range.

5. Price trends:
- In previous auctions, hammer prices have been significantly lower than the starting prices in many cases, suggesting that buyers may not be willing to pay the full starting price.
- However, there are examples of coins that have sold for much higher prices, indicating that there is potential for a high hammer price if the coin is of high quality.

### Estimation of the maximum purchase price:
Given that the starting price is 500 euros and considering the hammer price history, a reasonable maximum purchase price could be in the range of 600 to 700 euros, assuming that the coin has an acceptable condition and there is interest in the auction.

Therefore, an estimated maximum purchase price could be approximately 650 euros. This allows for a margin for competition in the auction, but it is also based on the trend of previous hammer prices.