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

1. Current Condition: The coin has a condition rating of 30 (BC+), which indicates that it is not in very good shape. This generally reduces its value compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins in better condition (70 and 75) has been approximately 104.74 euros.
- Previous auctions of this same coin have shown hammer prices that vary, but generally, coins in condition 70 have reached hammer prices of up to 160 euros.

3. Starting Price and Average Starting Price:
- The current starting price is 15 euros, which is significantly lower than the average starting price of 76.07 euros. This could indicate a buying opportunity at a low price.

4. Sales Trends: The coin has been sold in 39 out of 44 auctions, indicating good demand.

Given that the coin's condition is 30, and considering the hammer price history of coins in better condition, an estimated maximum purchase price could be set at a lower range than the average hammer price of coins in better condition.

Estimated Maximum Purchase Price:
We could estimate a maximum purchase price around 50-70 euros, taking into account the current condition and price history. This would allow for a reasonable purchase without exceeding the market value for a coin in this condition.