To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the starting price, the average hammer price, and previous prices from similar auctions.

1. Current Data:
- Starting price: €300
- Condition of this coin: 75 (MBC)
- Average hammer price: €398.09

2. Analysis of Previous Prices:
- The coin has been auctioned 27 times, of which it has sold 23 times.
- The average starting price of previous auctions is €261.85, and the average hammer price is €398.09.
- Looking at previous hammer prices, there are examples of hammer prices that vary significantly, especially for coins in better condition (for example, a hammer price of €1200 for a coin in condition 95).

3. Comparison of Conditions:
- The condition of 75 (MBC) is lower than the higher conditions that have reached significant hammer prices. However, there are examples of coins with similar conditions that have sold for reasonable prices (for example, €340 and €290).

4. Estimation of Maximum Price:
- Given that the average hammer price is €398.09 and considering that the condition of this coin is 75, a reasonable hammer price could be in the range of €300 to €450.
- Taking into account the starting price of €300 and the interest from collectors, a maximum purchase price could be around €450.

Conclusion: Based on the analysis of the data, an estimated maximum purchase price for this coin could be approximately €450.