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

1. Current Condition: The coin is in a condition of 90 (EF), indicating that it is in good shape. This is a positive factor that can increase its value.

2. Auction History:
- The average hammer price for similar coins in the past is 172.79 euros.
- In previous auctions, the highest hammer price for a coin with a similar condition (90) was 400 euros, which was the hammer price at the March 2023 auction.
- It is also noted that in previous auctions, the hammer price has varied, but it has reached up to 1400 euros in the case of a condition of 95.

3. Starting Price: The starting price of 400 euros is an indicator that the auction house expects significant interest in this coin.

4. Sales Trends: The coin has been sold in 40 out of 43 auctions, indicating good demand.

5. Comparison with Previous Prices: In previous auctions, the hammer price has fluctuated, but the price range for similar conditions suggests that the price could be in the range of 400 to 600 euros, especially considering that the current condition is 90.

Based on these factors, an estimated maximum purchase price for this coin could be between 500 and 600 euros. This takes into account the good condition, the price history, and market expectations.