To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Starting price: 50 euros.
2. Current condition: 30 (BC+), which is a relatively low condition.
3. Auction history:
- In 2020, a coin with a condition of 20 was auctioned with a hammer price of 75 euros.
- In 2022, a coin with a condition of 70 was auctioned with a hammer price of 115 euros.

4. Average hammer price: 86.67 euros, indicating that, in general, coins of this reference tend to sell for a price higher than the starting price.

Given that the current condition of the coin is 30, which is better than the 20 from the 2020 auction but significantly lower than the 70 from the 2022 auction, we can infer that the hammer price could be in an intermediate range.

Considering the hammer price of 75 euros for the condition of 20 and 115 euros for the condition of 70, we can estimate that the hammer price for a condition of 30 could be around 85-90 euros.

Therefore, an estimated maximum purchase price for this coin could be approximately 90 euros. This is based on the price history and the current condition, taking into account that the starting price is 50 euros, which suggests there is room for a higher bid.