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+). This indicates that it is not in the best condition, which generally affects its value.

2. Auction History: By observing the hammer prices of coins with a similar condition (30) in the past, we can see that the hammer prices have varied. For example:
- In previous auctions, coins with a condition of 30 have reached hammer prices ranging from 10 to 20 euros.
- The average hammer price for coins with higher conditions (such as 70 or 80) has been significantly higher, but this cannot be directly applied to a coin with a condition of 30.

3. Starting Price and Average Hammer Price: The current starting price is 12 euros, and the average hammer price for similar coins is 98.74 euros, but this includes coins in better condition.

4. Recent Trends: In the most recent auctions, the hammer prices for coins with a condition of 30 have been in the range of 15 to 30 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 20 and 25 euros. This is based on the analysis of past auctions and the current condition of the coin.