To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition rating of 25 (BC), indicating that it is in a fairly deteriorated state. This generally reduces its value compared to coins in better condition.

2. Historical Prices: By looking at previous auction prices, we can see that there is a great deal of variability. For example, in recent auctions, hammer prices have reached very high figures (such as 2800 euros in March 2023), but those prices correspond to coins in better condition (like 95 or 85 in grading).

3. Average Hammer Price: The average hammer price for this coin is 529 euros, which can serve as a reference point.

4. Starting Price: The current starting price is 350 euros, indicating that the seller expects a reasonable price for the coin.

5. Market Trends: Given that the coin has been auctioned 31 times and has had a sell-through rate of 97% (30 out of 31), this suggests that there is consistent interest in this coin, which may influence its price.

Taking all these factors into account, and considering that the coin's condition is low, an estimated maximum purchase price could be between 300 and 400 euros. This is based on the starting price and the average hammer price, adjusted for the low condition.

Therefore, a reasonable maximum purchase price could be approximately 400 euros.