To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors:

1. Starting price: 90 euros.
2. Average hammer price: 98.9 euros.
3. Auction history: The coin has been auctioned 1,295 times, of which it has sold 998 times, indicating good demand.
4. Previous prices: Previous hammer prices varied considerably, but there are examples of auctions reaching up to 650 euros in higher conservation conditions.

Since there is no information about the condition of the coin in question, it is prudent to consider a hammer price that is above the starting price and the average hammer price, but without reaching the extremes of the highest prices observed in the past.

Estimated maximum purchase price:
- Considering the starting price and the average hammer price, a reasonable maximum price could be approximately 120 to 150 euros. This is based on the trend of previous hammer prices and the observed demand.

Therefore, an estimated maximum purchase price could be 150 euros.