To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 70 (VF-), indicating that it is in acceptable shape, but not in the best condition. This may influence the price, as coins in better condition typically fetch higher prices.

2. Starting Price and Average Hammer Price: The starting price is 300 euros, and the average hammer price in previous auctions is 304.08 euros. This suggests that the hammer price tends to be close to the starting price.

3. Auction History: The coin has been auctioned 15 times, of which it has sold 13 times. This indicates good demand. Additionally, previous hammer prices varied, but there are examples of auctions reaching up to 400 euros in similar conditions.

4. Recent Trends: In the most recent auctions, hammer prices have ranged from 275 to 400 euros, suggesting sustained interest in the coin.

5. Previous Estimates: In past auctions, the estimated price has reached up to 600 euros, although this may be for coins in better condition.

Based on these factors, an estimated maximum purchase price could be in the range of 350 to 400 euros, considering that the starting price is 300 euros and that the auction history suggests buyers are willing to pay more for coins in similar conditions.

Therefore, a reasonable maximum purchase price could be approximately 400 euros.