To estimate a maximum purchase price for the coin in question, several factors can be considered, including the starting price, the condition of the coin, and the hammer prices of similar coins in the past.

1. Starting price: 12 euros.
2. Condition of the coin: 30 (BC+), indicating a low condition.
3. Previous hammer prices: Coins of the same reference have been sold in different conditions. The highest hammer prices for coins with similar conditions (30) range from 30 to 60 euros in recent auctions.

Given that the condition of the coin is low (30), and considering that the starting price is 12 euros, a reasonable maximum purchase price could be in the range of 30 to 50 euros, depending on the demand at the auction and the competition from other bidders.

Therefore, an estimated maximum purchase price could be 50 euros.