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

1. Coin Condition: The coin has a condition rating of 30 (BC+), which indicates that it is not in the best condition. This will affect its value compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins has been 166.58 euros.
- In the past, coins with a similar condition (30) have had hammer prices that varied, but generally, hammer prices for coins in higher conditions (70 or more) have been significantly higher.

3. Starting Price and Previous Prices:
- The current starting price is 15 euros, which is relatively low.
- In previous auctions, coins with a condition of 30 have had hammer prices ranging from 15 to 55 euros, depending on demand and other factors.

4. Sales Trends: The coin has been auctioned 123 times, with a high sales percentage (110 out of 123). This suggests that there is consistent interest in this coin.

5. Comparison with Previous Prices: In previous auctions, the hammer price for coins with a condition of 30 has been up to 55 euros, but there have also been cases where the price was lower.

Based on these factors, an estimated maximum purchase price for this coin could be between 40 and 55 euros, considering that the condition is not optimal and that interest in the coin may vary. Therefore, a reasonable maximum purchase price could be 50 euros.