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

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, many of which have higher conditions (90 and 95).

2. Previous Auction Prices: By observing previous auction prices, we can see that coins with higher conditions have reached auction prices of up to 260 euros. However, coins with lower conditions, like the one we are evaluating, tend to have lower auction prices.

3. Average Auction Price: The average auction price for this coin is 100.28 euros, indicating that, in general, coins of this reference tend to sell around this price.

4. Auction Trends: The coin has been auctioned 30 times, of which it has sold 27 times. This suggests that there is a consistent demand for this coin, although the auction price can vary significantly depending on the condition.

5. Comparison with Starting and Estimated Prices: The current starting price is 12 euros, and the average starting price is 45.79 euros. This suggests that the current starting price is quite low compared to the average.

Given all this, and considering that the condition of the coin is 30 (BC+), an estimated maximum purchase price could be in the range of 40 to 60 euros. This is based on the fact that, although the coin has a low starting price, its auction value may be higher, but it will not reach the levels of higher conditions.

Therefore, a reasonable estimated maximum purchase price could be 50 euros.