To estimate a maximum purchase price for the coin with a grade of 80 (MBC+), we can analyze historical data from previous auctions of coins with similar grades and their hammer prices.

1. Hammer Price Analysis:
- In previous auctions, coins with grades close to 80 have had varying hammer prices. For example:
- A coin with a grade of 80 was sold for 19 euros.
- Another with a grade of 85 was sold for 20 euros.
- Coins with a grade of 90 have reached hammer prices of up to 35 euros.

2. Price Trends:
- The average hammer price for coins of similar grade (MBC+) seems to be in the range of 15 to 25 euros, with some exceptional cases reaching up to 35 euros.

3. Additional Considerations:
- The starting price of 15 euros suggests that the auction house expects moderate interest in the coin.
- Given that there are 3 similar coins that will be auctioned in the coming days, this could influence competition and, therefore, the final price.

Estimation of Maximum Purchase Price:
Based on the previous analysis, a reasonable maximum purchase price for this coin, considering its grade and historical hammer prices, could be between 25 and 30 euros. This allows for a margin for competition in the auction and the interest of other collectors.