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

1. Current starting price: 750 euros.
2. Coin condition: 70 (MBC-), indicating an acceptable but not excellent condition.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 450 euros and sold for 700 euros.
- In 2015, the starting price was 600 euros and it sold for 850 euros.
- In 2013, the starting price was 650 euros and it sold for 675 euros, with a condition of 80.

4. Average prices:
- Average starting price: 612.5 euros.
- Average final price: 741.67 euros.

Given that the coin has a condition of 70, which is lower than the 2013 auction (80), it is likely that its final price will not reach the higher levels of previous auctions. However, the current starting price is higher than previous starting prices, suggesting that there is growing interest in this coin.

Taking all these factors into account, an estimated maximum purchase price could be around 800-850 euros. This is based on the highest final price from previous auctions and the current interest in the coin, but also considering the condition that could limit the final price.