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 very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- The coin has been auctioned 6 times in the past, of which it has sold 4 times. This suggests that there is interest in the coin.
- The average hammer price in previous auctions is 49 euros, which can serve as a good indicator of market value.

3. Previous Starting and Hammer Prices:
- In previous auctions, starting prices have varied, but the highest recorded hammer price is 135 euros (in 2014), although the condition of that coin is not specified.
- The most recent auction (2023) shows a hammer price of 20 euros for a coin with a condition of 100, indicating that the price can vary significantly based on condition.

4. Current Starting Price: The starting price is 40 euros, which is reasonable given the price history.

Based on these factors, an estimated maximum purchase price could be positioned between the average hammer price (49 euros) and a bit more, considering the good condition of the coin.

Therefore, a reasonable maximum purchase price could be 60 euros. This allows for some margin for competition in the auction, given that the condition is high and there is a history of interest in the coin.