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

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

2. Previous Auction Prices: By looking at previous auction prices for coins with a similar condition (30), we can see that auction prices have varied, but they have generally remained in a low range. For example, in past auctions, the hammer prices for coins with a condition of 30 have been between 12 and 15 euros.

3. Average Hammer Price: The average hammer price for this coin is 137.95 euros, but this includes coins in better conditions. Therefore, it is not a good indicator for our particular coin.

4. Recent Trends: In the most recent auctions, hammer prices for coins with a similar condition have been in the range of 12 to 20 euros.

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

Based on these factors, an estimated maximum purchase price for this coin, considering its condition and previous auction prices, could be in the range of 15 to 20 euros. This reflects a price that is reasonable given its state and market trends.