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

1. Current Condition: The coin has a condition rating of 30 (BC+), indicating that it is not in very good shape. This generally reduces its value compared to coins in better condition.

2. Previous Auction Prices: By observing the auction prices of similar coins in the past, we can see that prices have varied considerably. For example, coins with similar conditions have had auction prices ranging from 10 to 50 euros, depending on demand and other factors.

3. Average Auction Price: The average auction price for this coin is 84.28 euros, but this includes coins in better condition. Given that the condition of this coin is lower, the auction price is likely to be lower.

4. Recent Trends: In the most recent auctions, the auction prices for coins with similar conditions have been in the range of 20 to 35 euros.

5. Starting Price: The starting price is 12 euros, suggesting that the auctioneer expects moderate interest in the coin.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 30 and 40 euros. This reflects a balance between the current condition, previous auction prices, and market trends.