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 30 (BC+), indicating that it is not in very good shape. This generally negatively affects the auction price.

2. Previous Prices: By looking at the auction prices from previous sales, we can see that prices have varied considerably depending on the condition. For example:
- In 2020, a coin with a condition of 85 sold for 320 euros.
- In 2019, a coin with a condition of 70 sold for 280 euros.
- In 2021, a coin with a condition of 80 sold for 340 euros.

3. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price in previous auctions is 269.5 euros. This suggests that the starting price is relatively high compared to the average hammer price.

4. Auction Trends: Since the coin has sold every time it has been auctioned, this indicates a consistent interest in the coin, which could help maintain the price.

Based on these factors, and considering that the condition of the coin is quite low (30), an estimated maximum purchase price could be in the range of 200 to 250 euros. This is based on the fact that, although the starting price is 350 euros, the condition and previous hammer prices suggest that a buyer may not be willing to pay more than 250 euros for a coin in this state.

Therefore, an estimated maximum purchase price could be 250 euros.