To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting price: 862.88 euros. This is the initial price of the auction.
2. Auction history: The coin has been auctioned 26 times, of which it has sold 22 times. This indicates good demand.
3. Average hammer price: 487.11 euros. This is the average price at which the coins have sold in the past, which can be a good indicator of the real market value.
4. Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. Coins in better condition tend to reach higher prices.

Since the starting price is significantly higher than the average hammer price, this suggests that the seller has high expectations. However, the history of successful sales and the good condition of the coin may justify a higher price.

Taking all these factors into account, an estimated maximum purchase price could be set between the average hammer price and 20-30% above it, given the condition and historical demand.

Therefore, an estimated maximum purchase price could be:

Estimated maximum purchase price: 600 - 650 euros.

This range considers the good condition and demand, but also remains below the starting price to reflect market reality.