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

1. Current starting price: 300 euros.
2. Coin condition: 85 (ebc-), which is a fairly good condition, although not the best.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 600 euros and sold for 850 euros, with a condition of 95.
- In 2021, it was auctioned with an estimated price of 500 euros and sold for 400 euros, with a condition of 80.

4. Average starting and final prices: The average starting price is 450 euros and the average final price is 750 euros.

Given that the condition of the current coin (85) is lower than that of the 2020 auction (95) but higher than that of 2021 (80), we can infer that the final price should be somewhere in between the previous final prices, adjusted for condition.

### Estimation:
- The final price of the coin in 2020 was 850 euros, and in 2021 it was 400 euros.
- Since the current condition is better than that of 2021 but worse than that of 2020, we could estimate a final price in a range between 500 and 750 euros.

### Conclusion:
Taking all these factors into account, an estimated maximum purchase price for the coin could be around 600 to 700 euros. This is based on the auction history and the current condition of the coin.