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 not in very good shape. This will affect its value compared to coins in better condition.

2. Previous Prices: By looking at previous auction prices, we can see that the coin has reached auction prices of up to 480 euros in much better conditions (such as in 2021 with a condition of 85). However, in conditions similar to the current one (condition of 30), the lowest auction price was 90 euros in 2019.

3. Starting Price and Average Auction Price: The current starting price is 75 euros, and the average auction price in the past has been 265.71 euros. However, given that the current condition is quite low, it is unlikely that this average will be reached.

4. Auction Trends: Since the coin has sold every time it has been auctioned, this suggests that there is consistent interest in the coin, although the condition may limit the price.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 90 to 150 euros, considering that the lowest auction price in similar conditions was 90 euros and that interest in the coin could allow for a slight increase in price.

Therefore, a reasonable maximum purchase price could be 150 euros.