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

1. Current Condition: The coin has a condition of 70 (MS-), indicating that it is in an acceptable state, but not among the best qualities.

2. Auction History:
- The average hammer price for coins with a similar condition (70) has been approximately 30 euros in previous auctions.
- In past auctions, the hammer price has varied, but it has reached up to 65 euros in an auction with a similar condition (although the exact state of the coin is not specified).

3. Starting Price and Average Hammer Price:
- The current starting price is 20 euros, and the average hammer price in the past has been 31.73 euros. This suggests that the hammer price could be in a similar range or slightly higher.

4. Price Trends:
- In recent auctions, the hammer price has shown a trend of being in the range of 25 to 65 euros, depending on the condition and demand.

Based on these factors, an estimated maximum purchase price for this coin could be between 40 and 50 euros, considering that the condition is 70 and that the price history shows it has reached higher prices in the past. However, since the starting price is 20 euros, it is likely that the hammer price will not significantly exceed this range.

Therefore, a reasonable maximum purchase price could be 45 euros.