To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the current starting price, the coin's condition, and the hammer prices from previous auctions.

1. Current starting price: 250 euros.
2. Condition of the coin: 70 (MBC-), indicating a medium-low condition.
3. Average hammer price in the past: 314.2 euros.
4. Previous hammer prices:
- In auctions with similar condition (70), hammer prices have varied, but there are examples of auctions reaching up to 600 euros (in the case of a condition of 70 in 2016).
- In auctions with conditions of 75 and 80, hammer prices have been 600 and 400 euros respectively.

Given that the condition of the coin is 70, and considering that the average hammer price is 314.2 euros, we can estimate that the maximum purchase price could be in a range above the starting price, but not as high as the hammer prices of higher conditions.

Estimation of the maximum purchase price:
- Based on the average hammer price and previous hammer prices, a reasonable maximum price could be between 350 and 450 euros, considering that the condition is 70 and there is a history of hammer prices that justifies this estimation.

Therefore, an estimated maximum purchase price could be approximately 400 euros.