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 30 (BC+), which indicates that it is in quite low condition compared to other coins that have been auctioned in the past. This will negatively affect its value.

2. Historical Prices: By looking at previous auction prices, we can see that coins in better condition (higher condition values) have reached significantly higher prices. For example:
- In 2020, a coin with a condition of 85 sold for 800 euros.
- In 2019, a coin with a condition of 95 did not sell, but had an estimated price of 750 euros.
- In 2021, a coin with a condition of 95 sold for 650 euros.

3. Starting Price and Average Price: The current starting price is 150 euros, and the average starting price in previous auctions is 313.33 euros, with an average hammer price of 413.45 euros. This suggests that, in general, coins of this reference tend to sell for more than their starting price.

4. Sales Trends: The coin has been auctioned 12 times, of which 11 have sold. This indicates good demand, although the current condition may limit interest.

5. Comparison with Similar Condition Prices: The last auction of a coin with a condition of 70 sold for 252 euros. This can serve as a benchmark for the current condition of 30.

Taking all these factors into account, an estimated maximum purchase price for the coin with a condition of 30 could be in the range of 100 to 150 euros, given that the condition is quite low and interest may be limited. However, if there is competition in the auction, it could reach up to 200 euros, but that will depend on the demand at the time of the auction.

Therefore, a reasonable maximum purchase price could be 150 euros.