To estimate a maximum purchase price for the coin with a grade of 80 (MBC+), we can analyze the provided data.

1. Starting price: 40 euros.
2. Average auction price: 52.95 euros.
3. Coin grade: 80 (MBC+).
4. Auction price history for similar grades:
- In the past, coins with similar grades (such as 85 and 90) have reached auction prices ranging from 40 to 320 euros, depending on demand and other factors.
- For example, a coin with a grade of 85 sold for 120 euros, and another with a grade of 90 sold for 130 euros.

Since the grade of the coin in question is 80, which is lower than the grades of 85 and 90, we can infer that the auction price should be lower than the previous examples.

5. Estimation:
- Based on the average auction price (52.95 euros) and considering that the grade is slightly lower, a reasonable auction price could be in the range of 40 to 70 euros.
- Given that the starting price is 40 euros, a maximum purchase price could be slightly higher than the average auction price, but not so much as to exceed the range of similar grades.

Estimated maximum purchase price: 65 euros.

This price reflects a balance between the coin's grade and the historical auction prices for coins of similar grade.