To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous auction prices: By looking at previous hammer prices for coins with a similar condition (90), we can see that the hammer prices have varied, but they have generally been in a range between 300 and 400 euros.

3. Starting price and average hammer price: The current starting price is 400 euros, and the average hammer price for this coin has been 326.93 euros. This suggests that the starting price is reasonable, but it also indicates that there could be competition in the auction that might drive the price up.

4. Auction trends: The coin has been auctioned 101 times, with a high sales percentage (92 out of 101). This suggests that there is consistent interest in this coin, which could increase its value at auction.

5. Recent hammer prices: In the most recent auctions, hammer prices for coins of similar condition have been in the range of 400 to 650 euros, indicating that there is potential for the hammer price to be higher in this auction.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 450-500 euros. This range considers the good condition of the coin, the interest in it, and recent hammer prices.