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 in fairly low condition. This will negatively affect its value compared to coins in better condition.

2. Auction History:
- In the past, coins of the same reference with higher conditions (85 and 75) have been auctioned for 800 and 440 euros, respectively.
- The last auction with a condition of 75 had a hammer price of 440 euros, suggesting that the value of the coin decreases significantly with a lower condition.

3. Starting Price and Average Prices:
- The current starting price is 300 euros, and the average hammer price in previous auctions has been 620 euros, but this refers to coins in better condition.

4. Sales Trends: The coin has sold in 3 of the last 4 auctions, indicating interest in the coin, although the current condition may limit the price.

Given that the condition of the coin is quite low (30), and considering the hammer prices of coins in better condition, an estimated maximum purchase price could be set at a lower range.

Estimated Maximum Purchase Price:
Taking all these factors into account, a reasonable maximum purchase price could be between 150 and 250 euros. This is based on the need to adjust the value downward due to the condition and the interest shown in previous auctions.