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

1. Condition: The coin is in a condition of 100 (Uncirculated), indicating that it is in excellent shape. This generally increases its value compared to coins in lower conditions.

2. Auction history:
- The current starting price is 30 euros.
- The average hammer price in previous auctions for coins in similar condition (100) has been 65 euros (in the auction on 2020-12-17).
- In other auctions, hammer prices for coins in condition 95 have reached up to 44 euros, which also indicates that a coin in better condition can fetch higher prices.

3. Price trends:
- The average hammer price in previous auctions is 16.46 euros, but this includes coins in lower conditions. For coins in Uncirculated condition, the price can be significantly higher.
- The price trend in previous auctions shows that coins in Uncirculated condition have reached hammer prices of up to 65 euros.

4. Supply and demand: The coin will be auctioned in the coming days, which may increase competition and, therefore, the hammer price.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 65-70 euros, given that this is the highest price reached in previous auctions for coins in similar condition. However, it is important to consider that the final price may vary depending on the competition in the auction and the interest of collectors.