To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 80 (MBC+). This is an important factor, as coins in better condition tend to fetch higher prices.

2. Starting Price and Average Hammer Price: The starting price is 100 euros, and the average hammer price in previous auctions is 136.65 euros. This suggests that, in general, coins of this reference tend to sell for above their starting price.

3. Sales History: The coin has been auctioned 242 times, of which 221 have been sold. This indicates good demand and liquidity for this coin.

4. Previous Prices: By looking at the hammer prices of coins with similar conditions (around 80), we can see that they have reached hammer prices that vary, but some have gone for 150 euros or more in similar conditions.

5. Recent Trends: In the most recent auctions, hammer prices have shown an increase, which could indicate an upward trend in the value of this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 150 to 200 euros. This is based on the historical hammer prices for coins of similar condition and the price trend in recent auctions.

Therefore, a reasonable maximum purchase price could be 200 euros.