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

1. Current 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 in lower conditions.

2. Auction History:
- The average hammer price in previous auctions is 259 euros.
- In the most recent auction (2022-03-17), the coin was sold for 400 euros, indicating significant interest in this coin in good condition.
- The auction on 2022-04-28 also shows a hammer price of 360 euros for a coin with a condition rating of 85.

3. Current Starting Price: The starting price is 200 euros, which is below the average hammer price in previous auctions.

4. Sales Trends: The coin has been auctioned 12 times, of which it has sold 10 times, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 350 to 400 euros, given the demonstrated interest in previous auctions and the good condition of the coin. Therefore, a reasonable maximum purchase price could be 400 euros.