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

1. Starting price: 1200 euros.
2. Average auction price in the past: 2400 euros. This indicates that, in general, coins of this reference tend to sell for a significantly higher price than their starting price.
3. Condition: The coin has a condition of 30 (BC+), which indicates that it is not in the best condition. This may negatively affect its value compared to coins in better condition.
4. Auction history: It has been auctioned once and sold on that occasion, suggesting that there is interest in the coin.

Given that the average auction price is double the starting price, and considering the condition of the coin, an estimated maximum purchase price could be situated between the starting price and the average auction price.

Taking into account the condition (which could reduce the value compared to coins in better condition), a reasonable maximum purchase price could be around 1800 to 2000 euros. This reflects interest in the coin, but also takes into account its state of preservation.

Therefore, an estimated maximum purchase price could be 2000 euros.