To estimate a maximum purchase price for the coin that will be auctioned, we can consider several factors based on the provided data:

1. Starting price: 250 euros.
2. Auction history: The coin has been auctioned 1,295 times, of which it has sold 998 times. This indicates good demand.
3. Average hammer price: 98.9 euros, suggesting that the hammer price is usually significantly lower than the starting price.
4. Previous prices: There are several hammer prices in the past that vary considerably, with some reaching up to 650 euros in recent auctions.

Since there is no information about the condition of the current coin, it is difficult to make an accurate estimate. However, considering the starting price and the history of hammer prices, a reasonable maximum purchase price could be a multiple of the starting price, keeping in mind that previous hammer prices have been lower.

Estimated maximum purchase price:
- A conservative approach could be to set the maximum price around 300-400 euros, given that the starting price is 250 euros and considering that higher prices have been reached in previous auctions, but it is also important to note that the average hammer price is much lower.

Therefore, an estimated maximum purchase price could be 350 euros.