To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition rating of 75 (MBC), indicating that it is in fairly good shape, but not among the best qualities.

2. Auction History:
- The average hammer price in previous auctions for coins of similar condition (75-85) has varied, but there are examples where hammer prices have reached up to 35 euros (as in the auction on 2020-12-15).
- Most previous auctions show that the hammer price tends to be between 15 and 35 euros for similar conditions.

3. Starting Price and Average Hammer Price:
- The current starting price is 20 euros, and the average hammer price in previous auctions is 23.76 euros.

4. Recent Trends: In the most recent auctions, the hammer price has been around 20-45 euros, depending on the condition and demand.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 30 to 40 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate.

Therefore, a reasonable maximum purchase price could be 35 euros.