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: 250 euros.
2. Coin condition: 85 (ebc-), which is good condition, although not the best.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 900 euros and sold for 1800 euros (condition 90).
- In 2024, the starting price was 600 euros and it sold for 2000 euros (condition 80).

4. Average starting and final prices:
- Average starting price: 583.33 euros.
- Average final price: 1410 euros.

Given that the condition of the current coin (85) is lower than the conditions of the previously auctioned coins (90 and 80), it is reasonable to expect that the final price will not reach the highest levels observed, but it could still be significant.

### Estimation of the maximum purchase price:
- Considering the average final price (1410 euros) and adjusting for condition, we could estimate that the maximum purchase price could be in a range between the average final price and 20-30% less due to the condition.

Therefore, a reasonable maximum purchase price could be between 1000 and 1200 euros.

Final estimation: 1100 euros as the maximum purchase price.