To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, including the starting and final prices of previous auctions, as well as the frequency of auctions and sales.

1. Current starting price: 70 euros.
2. Auction history: The coin has been auctioned 88 times, of which it has sold 76 times. This indicates a good sales rate.
3. Previous prices:
- The average final price in previous auctions is 21.65 euros.
- The highest final prices in previous auctions have reached up to 85 euros (on 2020-09-17).
4. Condition: There is no information about the condition of this coin, which may affect its value. However, in previous auctions, coins with high condition (100) have achieved higher final prices.

Given that the starting price is significantly higher than the previous average final price, and considering that the starting price is 70 euros, it is reasonable to assume that the maximum purchase price could be in a higher range, but it will not necessarily reach the starting price.

Estimation of the maximum purchase price:
- Based on the history of final prices, a reasonable maximum purchase price could be between 50 and 70 euros, considering that the starting price is 70 euros and the average final price is 21.65 euros.

Therefore, an estimated maximum purchase price could be between 50 and 70 euros, depending on the demand at the auction and the condition of the coin.