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

1. Current coin data:
- Starting price: €12
- Grade: 80 (MBC+)

2. Relevant historical data:
- The average hammer price for coins with a similar grade (80) has varied, but it can be observed that in several previous auctions, the hammer prices have ranged from €15 to €30.
- For example, in previous auctions with a grade of 80, the hammer prices have been:
- €15 (2018 auction)
- €20 (2022 auction)
- €25 (2022 auction)
- €30 (2022 auction)

3. Hammer price analysis:
- The average hammer price for coins with a grade of 80 seems to be around €20-25.
- The trend shows that, although there are variations, the hammer price tends to be higher than the starting price.

4. Estimation of the maximum purchase price:
- Given that the average hammer price for coins with a grade of 80 is in the range of €20-30, and considering that the starting price is €12, a reasonable maximum purchase price could be slightly higher than the average hammer price.

Therefore, an estimated maximum purchase price for this coin could be €25. This is based on the trend of previous hammer prices and the grade of the coin.