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 (EBC), 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 2021, a coin with a condition of 90 was auctioned for 440 euros.
- In 2022, a coin with a condition of 95 was auctioned for 700 euros.
- In 2014, a coin with a condition of 95 was also auctioned for 700 euros.

3. Average Auction Price: The average auction price for this coin is 211.33 euros, giving us an idea of what buyers are willing to pay on average.

4. Starting Price: The starting price is 250 euros, indicating that the seller expects a reasonable price for the coin.

5. Auction Trends: The coin has been auctioned 29 times, of which it has sold 27 times, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 600 euros, considering that the condition is good and there are precedents of successful sales at higher prices. However, since the starting price is 250 euros, it is likely that buyers will be willing to bid beyond this price, especially if there is interest in the coin.

Therefore, an estimated maximum purchase price could be approximately 500 euros.