To estimate a maximum purchase price for the coin with a grade of 30 (BC+), we can analyze historical data and price trends.

1. Data from previous auctions:
- The coin has been auctioned 101 times, of which it has sold 92 times.
- The average hammer price is 153.86 euros, but this value includes coins in better conditions.
- In previous auctions, hammer prices for coins in similar conditions (BC+) have varied, but they generally fall within a lower range.

2. Hammer prices for similar grades:
- In past auctions, hammer prices for coins with a grade of 30 (BC+) have been lower. For example, in a recent auction, a coin with a grade of 30 sold for 45 euros.
- Hammer prices for coins in grades of 70 and 80 have been significantly higher, indicating that the grade drastically affects the price.

3. Estimation based on starting price and average hammer price:
- The current starting price is 12 euros, suggesting that the seller expects moderate interest.
- Given that the average hammer price for coins in similar conditions has been approximately 20-45 euros, we can estimate that the maximum purchase price for this coin could be in the range of 30 to 50 euros.

Conclusion: Based on the grade of 30 (BC+) and historical data, an estimated maximum purchase price for this coin could be 45 euros.