To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially considering the current condition of the coin (30 BC+) and the auction prices of coins with similar conditions in the past.

1. Condition of the coin: The coin has a condition of 30 (BC+), which indicates that it is not in the best shape. This generally translates to a lower auction price compared to coins in better condition.

2. Previous auction prices: By observing the auction prices of coins with a similar condition (around 30), we find that the auction prices have varied, but are generally in a low range. For example:
- In previous auctions, coins with a condition of 30 have had auction prices ranging from 15 to 25 euros.
- In a recent auction (2022-09-14), a coin with a condition of 30 was sold for 15 euros.

3. Average auction price: The average auction price for this particular coin is 165.94 euros, but this includes coins in better condition. Therefore, it is not a good indicator for our estimation.

4. Starting price: The current starting price is 12 euros, which suggests that the seller has moderate expectations.

5. Estimation: Given that the condition of the coin is low and considering the previous auction prices for similar coins, an estimated maximum purchase price could be between 20 and 30 euros. This is based on the fact that buyers may be willing to pay a little more than the starting price if the coin has sentimental or collectible value.

Conclusion: An estimated maximum purchase price for this coin could be approximately 25 euros.