To estimate a maximum purchase price for the coin in question, let’s consider the following factors:

1. Starting Price: 700 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 30 (BC+), indicating that it is in fairly low condition. This generally negatively affects the price, as coins in better condition tend to fetch higher prices.
3. Auction History: The coin has been auctioned once in the past but did not sell, suggesting that interest in this coin may be limited or that the starting price is too high for its condition.
4. Average Hammer Price: There is no data on previous sales, making it difficult to estimate a realistic hammer price.

Since the coin did not sell in its first auction and considering its condition, it is likely that buyer interest is low. Therefore, a reasonable maximum purchase price could be a percentage of the starting price, adjusted for condition.

A conservative estimate could be between 50% and 70% of the starting price, given the condition. This leads us to a range of:

- 50% of 700 euros = 350 euros
- 70% of 700 euros = 490 euros

Thus, an estimated maximum purchase price could be around 400 to 490 euros. This would allow a potential buyer to take into account the risk that the coin may not sell in the future and its condition.