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

1. Current Condition: The coin has a condition of 70 (MBC-), indicating that it is in an acceptable state, but not of the highest quality.

2. Auction History:
- In the auction on October 5, 2021, a coin with a condition of 70 sold for 210 euros.
- In the auction on June 3, 2021, a coin with a condition of 30 sold for 220 euros, suggesting that condition has a significant impact on price.
- In the auction on September 27, 2022, a coin with a condition of 75 sold for 280 euros, indicating that better condition can increase the final price.

3. Starting and Final Prices: The current starting price is 110 euros, and the average final price in the past has been 242.5 euros. This suggests there is potential for an increase in the final price.

4. Average Final Price: The average final price of 242.5 euros is a good indicator of the market value of the coin.

Considering these factors, an estimated maximum purchase price could be in the range of 210 to 280 euros, taking into account that the current condition is 70 and that it has sold for similar prices in the past.

Therefore, a reasonable maximum purchase price could be 250 euros. This allows for a margin for competition in the auction and aligns with previous final prices.