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 (XF), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Historical Prices: By looking at previous auction prices for coins with similar conditions, we can see that:
- In 2018, a coin with a condition of 95 sold for 1700 euros.
- In 2019, a coin with a condition of 90 sold for 650 euros.
- In 2022, a coin with a condition of 85 sold for 400 euros.

3. Starting Price and Average Hammer Price: The current starting price is 300 euros, and the average hammer price in the past has been 284.88 euros. This suggests that the starting price is reasonable, but it also indicates that the hammer price may be lower than the starting price.

4. Sales Trends: The coin has sold in 33 out of the last 36 auctions, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 800 euros, considering that the condition is good and there are precedents for sales at higher prices for coins in similar conditions. However, since the starting price is 300 euros, it is likely that the hammer price will not significantly exceed this value unless there is strong competition in the auction.

Therefore, a reasonable maximum purchase price could be 700 euros.