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

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

2. Current Starting Price and Previous Prices: The current starting price is 425 euros. In previous auctions, coins with similar conditions (around 90) have had hammer prices that varied, but many have exceeded 300 euros. For example, in recent auctions, coins with a condition of 90 have reached hammer prices of up to 380 euros.

3. Price Trends: By observing the hammer prices of coins with similar conditions in the past, the maximum hammer price has been in the range of 350 to 400 euros on several occasions. However, the current starting price is higher, suggesting that the market may be valuing this coin more than in previous auctions.

4. Demand and Supply: Given that there are 3 auctions scheduled in the coming days, this may increase competition and, therefore, the final price.

Based on these factors, an estimated maximum purchase price for this coin could be between 400 and 450 euros, considering that the starting price is 425 euros and that the condition is good. This also takes into account the possibility that demand in the auction may drive up the final price.