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

1. Current Condition: The coin has a condition of 70 (MBC-), which indicates that it is in relatively good shape, but not among the best qualities. This may influence the price, as coins in better condition tend to fetch higher prices.

2. Previous Prices:
- In the 2020 auction, the coin sold for 150 euros with a condition of 30.
- In 2021, it sold for 120 euros, also with a condition of 30.
- In 2016, although it did not sell, the starting price was 350 euros, indicating that there were high expectations at that time.

3. Starting Price and Average Price: The current starting price is 110 euros, and the average hammer price in previous auctions is 135 euros. This suggests that the market expects the coin could sell for a price close to this average.

4. Demand and Supply: The coin has been auctioned 3 times in the past, of which it has sold 2 times. This indicates moderate interest in the coin, but also that there is a possibility it may not sell this time, given that there was one auction where it did not sell.

5. Market Trends: Since the average starting price is 166.67 euros, this suggests that the market is willing to pay more than the current starting price.

Taking all these factors into account, an estimated maximum purchase price could be around 150-180 euros. This is based on the previous hammer price and the current condition of the coin. However, since the starting price is 110 euros, it is possible that buyers may be willing to bid more, especially if there is interest in the coin.

Therefore, a reasonable maximum purchase price could be 180 euros.