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 in a relatively low state compared to coins that have been sold in the past with a condition of 70. This suggests that the hammer price could be lower than that of coins in better condition.

2. Previous prices: Previous hammer prices for coins with a condition of 70 have ranged from 21 to 28 euros. Given that the condition of the current coin is significantly lower, it is reasonable to expect that the hammer price will be below these values.

3. Average hammer price: The average hammer price for this coin in the past has been 24.5 euros. However, since the current condition is 30, it is likely that the hammer price will be below this average.

4. Starting price: The starting price is 12 euros, which may influence bidder interest.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 15 to 20 euros. This is based on the current condition and the previous hammer prices of coins in better condition. A reasonable maximum price could be 18 euros.