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 20 (BC-), indicating that it is in quite low condition. This generally reduces the value of the coin compared to coins in better condition.

2. Auction History:
- The average hammer price for similar coins has been 306.04 euros, but this includes coins in better condition.
- In previous auctions, hammer prices have varied significantly, with some reaching high prices (for example, 1400 euros in a 2016 auction, but that could be an exceptional case).

3. Starting Price and Estimated Price:
- The current starting price is 150 euros, and the average starting price in the past has been 127.33 euros. This suggests that the current starting price is reasonable.
- The estimated price for this auction is 150 euros, indicating that the auctioneers believe this is a fair value.

4. Sales Trends: The coin has been sold in 28 out of 31 auctions, indicating good demand, although the low condition may affect this.

5. Comparison with Previous Prices: In previous auctions, coins with similar conditions have had hammer prices that varied, but are generally below 200 euros.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 150 and 200 euros, considering that the condition is a limiting factor. A price of 180 euros could be a reasonable target, as it is in the middle of this range and takes into account historical demand and the current starting price.