To estimate a maximum purchase price for the coin in question, we can 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. Historical Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In 2022, a coin with a condition of 70 was auctioned for 400 euros.
- In 2024, a coin with a condition of 90 was auctioned for 750 euros.

3. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price in previous auctions is 298.06 euros. However, since the condition of this coin is above the average of previously auctioned coins, we can expect the hammer price to be higher.

4. Auction Trends: The coin has been auctioned 21 times, with an 85.71% sell-through rate (18 out of 21). This suggests that there is consistent interest in this coin, which may increase its value.

Based on these factors, an estimated maximum purchase price could be between 600 and 800 euros, considering the good condition and the hammer prices of similar coins in the past. A price of 750 euros could be a reasonable target, given the highest recorded hammer price for a coin of similar condition.