To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the context of the current auction.

1. Current starting price: 600 euros.
2. Average hammer price in the past: 678.48 euros.
3. Hammer price history:
- Hammer prices have varied, with some reaching up to 1200 euros in higher conservation conditions (such as 100 and 95).
- Most hammer prices fall between 550 and 850 euros, depending on the conservation.

4. Condition of the coin: There is no information on the current condition, which can significantly affect the price. However, by observing the hammer prices of coins with similar conditions in the past, we can make an estimation.

5. Price trends:
- In previous auctions, coins with conditions ranging from 80 to 95 have had hammer prices between 550 and 900 euros.
- The most recent auction shows a hammer price of 721 euros, suggesting that interest in this coin remains strong.

### Estimation of the maximum purchase price:
Given that the average hammer price is 678.48 euros and considering that the starting price is 600 euros, a reasonable maximum purchase price could be in the range of 800 to 900 euros, assuming the coin has an acceptable condition.

Therefore, an estimated maximum purchase price could be approximately 850 euros. This allows for a margin for competition in the auction and aligns with previous hammer prices.