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

1. Starting price: 50 euros.
2. Coin condition: 30 (BC+), indicating a low condition.
3. Auction history:
- In 2019, the coin with a condition of 30 was auctioned with a starting price of 60 euros and sold for 110 euros.
- In 2017, although the condition is not specified, the starting price was 75 euros and it sold for 80 euros.
- In 2024, again with a condition of 30, the starting price was 150 euros and it sold for 165 euros.

4. Average starting price: 83.75 euros.
5. Average selling price: 118.33 euros.

Given that the condition of the coin is 30 (BC+), and considering that in the past it has reached selling prices of up to 165 euros under similar conditions, we can make an estimation.

The current starting price is 50 euros, which is quite low compared to previous selling prices. However, since the condition is low, it is reasonable to adjust the maximum purchase price.

Estimation of the maximum purchase price:
- Based on the average selling price (118.33 euros) and considering the condition, a reasonable maximum purchase price could be in the range of 80 to 100 euros, given that the starting price is 50 euros and there is interest in the auction.

Therefore, an estimated maximum purchase price could be 100 euros.