To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current Condition: The coin has a condition of 75 (MBC), indicating that it is in quite good shape.

2. Auction History:
- The coin has been auctioned 15 times, of which it has sold 13 times.
- The average starting price in previous auctions is 31.58 euros, and the average hammer price is 47.17 euros.
- In previous auctions with a similar condition (75), the hammer prices have varied, but there are examples where it sold for 34 euros (2019) and 45 euros (2022).

3. Price Trends:
- In recent auctions, hammer prices for coins with conditions ranging from 70 to 80 have been in the range of 34 to 45 euros.
- The last auction with a hammer price of 21 euros for a coin with a condition of 70 suggests that the market may be a bit lower at this time.

4. Starting Price: The starting price is 20 euros, which is reasonable given the price history.

Based on this data, an estimated maximum purchase price could be in the range of 45 to 55 euros, considering that the condition is good and that the average hammer price in the past has been higher. However, given the market context and the recent auction with a lower hammer price, a more conservative price could be around 45 euros.

Therefore, an estimated maximum purchase price could be 45 euros.