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

2. Previous auction prices:
- In 2019, the coin was auctioned with a starting price of 200 euros and sold for 300 euros, with a condition of 75.
- In 2016, the starting price was 100 euros and it sold for 130 euros, but the condition is not specified.
- In 2014, the starting price was 90 euros and it sold for 100 euros, without specifying the condition.

3. Average hammer price: The average hammer price for this coin is 176.67 euros, which provides a good reference point.

4. Current starting price: The starting price is 90 euros, which is relatively low compared to previous hammer prices.

5. Market trends: Since the coin has sold in all previous instances and there have been no failed sale attempts, this suggests that there is consistent interest in the coin.

Taking all these factors into account, and given that the current condition is quite low, an estimated maximum purchase price could be in the range of 100 to 130 euros. This is based on the fact that, although the condition is not high, the interest in the coin and previous hammer prices suggest that it could reach a reasonable price at auction.

Therefore, an estimated maximum purchase price could be 120 euros.