To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 120 euros.
2. Coin condition: 30 (BC+), indicating that the coin has a relatively low state of preservation.
3. Auction history:
- In 2016, it was auctioned with a starting price of 25 euros and sold for 30 euros.
- In 2022, it was auctioned with a starting price of 18 euros and sold for 45 euros.
- In 2022, another auction had a starting price of 40 euros and sold for 55 euros, but with a condition of 70, which is significantly better.

4. Average hammer price: 62.5 euros, indicating that, in general, coins of this reference tend to sell for a price lower than the current starting price.

Given that the condition of the coin in the current auction is 30 (BC+), which is considerably lower than the condition of 70 from the last auction that had a hammer price of 55 euros, it is reasonable to assume that the hammer price for this coin will not reach the starting price of 120 euros.

Based on the hammer price history and the current condition, an estimated maximum purchase price could be between 40 and 60 euros. This aligns with the average hammer price and the fact that condition is an important factor in valuation.

Therefore, an estimated maximum purchase price could be 50 euros.