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 25 (BC), which indicates that it is in quite poor condition. This will negatively affect its value compared to coins in better condition.

2. Historical Prices:
- The average hammer price in previous auctions is 383.67 euros.
- However, the hammer prices from past auctions have varied, with some being significantly higher (for example, 600 euros in 2014) and others lower (such as 250 euros in 2017).

3. Starting Price and Estimated Price: The current starting price is 350 euros, and the estimated price is also 350 euros. This suggests that the seller expects the coin to sell at this price.

4. Sales Trends: The coin has sold every time it has been auctioned, indicating a consistent interest in it.

5. Condition of Similar Coins: In previous auctions, coins with higher conditions (such as 70 and 80) have reached higher hammer prices, suggesting that condition is a key factor in pricing.

Given that the condition of this coin is low (25), and considering the average hammer price of 383.67 euros, a reasonable maximum purchase price could be in the range of 150 to 200 euros. This is based on the need to adjust the price downward due to the low condition, despite the historical interest in the coin.

Therefore, an estimated maximum purchase price could be 200 euros.