Based on the provided data, here is an analysis to estimate a maximum purchase price for the coin in question:

1. Starting Price: 300 euros.
2. Condition: 70 (MBC-), which indicates a fairly good condition, although not the best possible.
3. Auction History: The coin has been auctioned once and sold on that occasion, suggesting there is interest in the coin at this price.

Since the average hammer price is equal to the starting price (300 euros) and considering that the condition is 70, we could estimate that the maximum purchase price could be slightly above the starting price, but not by much, as there are no historical data suggesting a significant increase in value.

Therefore, an estimated maximum purchase price could be between 300 and 350 euros, depending on the demand at the auction and the interest from collectors. However, it is advisable not to exceed 350 euros to avoid overpaying at an auction.