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 observing previous auction prices for coins with a similar condition (30), we can see that auction prices have varied, but generally remained in a low range. For example, in past auctions, the hammer prices for coins with a condition of 30 have been around 12 to 20 euros.

3. Average Hammer Price: The average hammer price for this coin is 137.95 euros, but this includes coins in better condition. For a condition of 30, it is unlikely that this price will be reached.

4. Recent Trends: In the most recent auctions, hammer prices for coins with a similar condition have been in the range of 20 to 40 euros.

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

Taking all these factors into account, an estimated maximum purchase price for this coin, considering its condition and previous hammer prices, could be in the range of 25 to 35 euros. This reflects a reasonable price that could be achieved at auction, given the sales history and the condition of the coin.