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

1. Relevant data from previous auctions:
- Coins with a grade of 80 (MBC+) have been auctioned several times. Some examples are:
- 2012-02-22: Hammer price 350 (grade 80)
- 2012-04-29: Hammer price not available (grade 80)
- 2012-12-01: Hammer price 373.37 (grade 80)
- 2019-05-29: Hammer price 680 (grade 75)
- 2023-03-22: Hammer price 360 (grade 75)

2. Price analysis:
- The highest hammer price for a grade of 80 (MBC+) was 680 euros.
- The lowest hammer price in this range was 350 euros.
- The general trend shows that hammer prices for similar grades range between 350 and 680 euros.

3. Additional considerations:
- The average hammer price for this particular coin is 1035.52 euros, indicating that, although there are examples of lower prices, the market may be willing to pay more for coins of this reference.
- Given that the coin has a starting price of 500 euros, it is reasonable to consider that the hammer price could be above this value, especially if there is interest in the auction.

4. Final estimation:
- Taking all these factors into account, an estimated maximum purchase price for this coin with a grade of 80 (MBC+) could be in the range of 600 to 800 euros. This is based on previous hammer prices and potential interest in the auction.

Therefore, a reasonable maximum purchase price could be 800 euros.