To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition 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. Previous Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In recent auctions, coins with a condition of 85 have sold for around 400-480 euros.
- A coin with a condition of 95 sold for 650 euros.

3. Starting Price and Average Hammer Price: The current starting price is 450 euros, and the average hammer price in the past has been approximately 396 euros. This suggests that the hammer price may be in a similar range or slightly above the starting price.

4. Auction Trends: The coin has been auctioned 25 times, with a sell-through rate of 96% (24 out of 25). This indicates a consistent interest in this coin, which may increase its value.

5. Estimation Based on Condition and Previous Prices: Given that the coin's condition is 90, and considering that coins with conditions of 85 and 95 have reached hammer prices of up to 480 and 650 euros respectively, we can estimate that the maximum purchase price could be in a range above the starting price.

Taking all these factors into account, an estimated maximum purchase price for this coin could be approximately 550-600 euros. This is based on the condition, previous hammer prices, and the trend of interest in the coin.