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 30 (BC+), indicating that it is not in the best condition. This generally reduces its value compared to coins in better condition.

2. Auction History:
- The average hammer price of similar coins in the past is 437.46 euros.
- The coin has been auctioned 18 times, of which it has sold 13 times. This indicates good demand, although there is also a significant number of coins that have not sold (5 times).

3. Previous Prices:
- In previous auctions, hammer prices have varied significantly, ranging from 90 euros to 1200 euros, depending on the condition and other factors.
- The last auction with a similar condition (30) had a hammer price of 500 euros.

4. Starting Price and Estimates:
- The current starting price is 40 euros, which is relatively low compared to the average hammer price of similar coins.
- The average starting price of 405.83 euros and the average hammer price of 437.46 euros suggest that there is potential for an increase in the hammer price.

Given all this, and considering that the condition of the coin is a limiting factor, an estimated maximum purchase price could be in the range of 150 to 250 euros. This is based on the hammer price history for coins with a similar condition and the general interest in the coin.

Therefore, a reasonable maximum purchase price could be 250 euros.