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 is relatively low compared to other coins that have been auctioned in the past, where many have higher conditions (70, 80, 90, etc.).

2. Auction History:
- The average hammer price for similar coins is 541.12 euros.
- The average starting price is 464.18 euros.
- In the past, coins with similar conditions have sold for prices that varied considerably, but generally, coins in better condition have reached much higher prices.

3. Previous Prices:
- Previous auctions of coins with conditions between 25 and 30 have had hammer prices ranging from 15 to 320 euros, depending on demand and the rarity of the coin.
- The last auction of a coin with a similar condition (30) had a hammer price of 56 euros.

4. Market Trends: Given that the coin has a starting price of 12 euros, it is likely that collector interest is not very high, which could limit the hammer price.

Based on these factors, an estimated maximum purchase price for this coin could be between 50 and 80 euros, considering that it is a coin with a low condition and that market interest may not be very high. However, if there is particular interest in this specific coin, the price could be higher, but it is unlikely to exceed 100 euros in this condition.