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

1. Current Condition: The coin has a condition of 30 (BC+), which indicates that it is not in very good shape. This generally negatively affects the auction price.

2. Auction History:
- The coin has been auctioned 8 times and has sold on all occasions, indicating consistent demand.
- Previous auction prices for coins with similar conditions (30 and 70) have varied, but the highest auction price for a condition of 30 was 50 euros in 2020.

3. Starting and Final Prices:
- The current starting price is 15 euros, which is below the average starting price of 30.38 euros. This could indicate an opportunity to buy at a lower price.
- The average final price for similar coins is 44.88 euros.

4. Price Trends:
- In previous auctions, final prices have ranged from 11 to 170 euros, depending on the condition. However, the highest prices correspond to higher conditions (such as 80 and 90).

Given that the condition of the coin is 30, and considering the auction price history, an estimated maximum purchase price could be in the range of 20 to 30 euros. This is based on the fact that, although the condition is not high, demand has been consistent and the average final price is 44.88 euros, suggesting that there could be a profit margin if purchased at a reasonable price.

Estimated maximum purchase price: 30 euros.