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

1. Current starting price: 250 euros.
2. Current condition: 75 (MBC).
3. Auction history:
- In 2015, it was auctioned with a starting price of 1800 euros and sold for 1950 euros.
- In 2021, it was auctioned with an estimated price of 500 euros and sold for 550 euros (condition 80).
- In 2022, it was auctioned with a starting price of 400 euros and sold for 700 euros (condition 85).

4. Average prices:
- Average starting price: 816.67 euros.
- Average selling price: 870 euros.

### Analysis

- The current condition of the coin (75) is lower than the conditions of the coins auctioned in 2021 (80) and 2022 (85), suggesting that its value may be lower than the selling prices of those auctions.
- However, the selling price of 1950 euros in 2015 is a significant data point, although the condition of that coin is not specified.
- The average selling price of 870 euros indicates that, in general, coins of this reference tend to sell for higher prices than the current starting price.

### Estimation

Given that the condition of the current coin is 75, which is lower than the conditions of previous auctions, and considering the average selling price, an estimated maximum purchase price could be in the range of 500 to 700 euros. This is based on the fact that the coin has a starting price of 250 euros, but its value could be considerably higher due to its auction history.

Estimated maximum purchase price: 650 euros.