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

1. Current starting price: 200 euros.
2. Current grade: 70 (MBC-), which is an acceptable grade, but not excellent.
3. Auction history:
- In the March 2023 auction, the coin started at 200 euros and sold for 400 euros with a grade of 90.
- In the May 2024 auction, it started at 250 euros and sold for 300 euros with a grade of 70.
- In the September 2024 auction, it started at 250 euros with an estimated price of 350 euros and a grade of 75.

4. Average hammer price: 350 euros, indicating that, in general, coins of this reference tend to sell for a price higher than the starting price.

5. Trends: The coin had a higher hammer price in the March 2023 auction, suggesting that there is significant interest in this reference, especially in better conditions.

Given that the current grade of the coin is 70, which is lower than that of the March 2023 auction (90), but similar to that of May 2024 (70), we can infer that the hammer price could be in a lower range than 400 euros, but it could still be competitive.

Estimation of the maximum purchase price:
- Considering the average hammer price of 350 euros and the grade of 70, a reasonable maximum purchase price could be between 250 and 300 euros, given that the starting price is 200 euros and there is interest in the coin.

Therefore, an estimated maximum purchase price could be 300 euros.