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

1. Coin Condition: The coin has a condition of 100 (MS), indicating that it is in very good shape. This generally increases its value compared to coins with lower conditions.

2. Auction History:
- The average hammer price in previous auctions is 21.81 euros.
- In recent auctions, hammer prices have reached up to 65 euros (in January 2023).
- The last auction where this coin with a condition of 100 was sold had a hammer price of 36 euros (in April 2022).

3. Starting Price and Estimates:
- The current starting price is 15 euros, which is below the historical average hammer price.
- The estimated price for the auction is 30 euros, suggesting that the coin is expected to reach a higher value.

4. Sales Trends: The coin has been sold in 26 out of the 29 previous auctions, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 30 to 65 euros, considering that the highest hammer price in the past was 65 euros and that the estimated price for the auction is 30 euros.

Therefore, a reasonable maximum purchase price could be 40-50 euros, taking into account the condition and sales history.