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 quite good shape.

2. Starting Price: The starting price is 400 euros.

3. Sales History:
- The average hammer price in previous auctions for coins of similar condition (EF) has been approximately 297.25 euros.
- In previous auctions, hammer prices for coins with conditions ranging from 85 to 95 have varied, with some reaching up to 400 euros or more, depending on demand and rarity.

4. Recent Trends: In the most recent auctions, hammer prices have shown an increase, especially for coins in good condition.

5. Comparison with Previous Prices:
- In previous auctions, coins with a condition of 90 have reached hammer prices ranging between 300 and 400 euros, with some exceptional cases exceeding 400 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 450 and 500 euros, considering that interest in coins in good condition may lead bidders to offer more than usual. However, given the starting price of 400 euros, it is likely that buyers will be willing to bid up to that range if demand is high.