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+), indicating that it is in a fairly deteriorated state. This will negatively affect its value compared to coins in better condition.

2. Previous Prices:
- In 2020, the coin was auctioned with a starting price of 150 euros and sold for 300 euros, with a condition of 70.
- In 2015, the starting price was 400 euros and it sold for 400 euros, with a condition of 85.

3. Average Hammer Price: The average hammer price for this coin in the past has been 328.33 euros, which provides a good reference point.

4. Current Starting Price: The current starting price is 230 euros, which is lower than the average hammer price.

Given that the current condition is significantly lower than in previous auctions, this suggests that the hammer price could be lower than the average. However, interest in the coin and market demand also play an important role.

Taking all these factors into account, an estimated maximum purchase price could be around 250-280 euros, considering that the condition is a limiting factor. This would allow for a competitive bid without exceeding the value it could reach at auction.