To estimate a maximum purchase price for the coin with a grade of 70 (MBC-), we can analyze historical data and price trends.

1. Current coin data:
- Starting price: €12
- Grade: 70 (MBC-)

2. Relevant historical data:
- In previous auctions, coins with a similar grade (70) have had hammer prices that vary, but are generally found in a range from €12 to €45.
- For example, in past auctions:
- 2020-05-26: Hammer price €15 (grade 70)
- 2019-07-03: Hammer price €14 (grade 70)
- 2022-09-14: Hammer price €25 (grade 70)
- 2023-04-19: Hammer price €26 (grade 70)

3. Analysis of hammer prices:
- The average hammer price for coins with a grade of 70 seems to be in the range of €15 to €26.
- The average hammer price of similar coins in the past has been approximately €20-25.

4. Estimation of maximum purchase price:
- Given that the starting price is €12 and considering that the hammer price for coins in grade 70 has been in the range of €20-26, 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 analysis of previous hammer prices and the current grade of the coin.